Introduction

This is part of a series of posts, Dungeon-A-Week, that I started a few weeks ago. This series contains short, single-level D&D 5E adventures that people can grab when they don’t have a lot of time and need something quick and fun to run that night.

Note that is an experiment—I have no idea if I will continue through all twenty levels or not, though I will make a serious attempt to complete this project.

In writing these adventures, I will keep the following design guidelines in mind:

• These adventures will most likely be a series of small dungeon environments. I don’t intend any of these to be event-based.
• The encounters in the adventure will be enough to provide a single level’s worth of experience points.
• Each dungeon will have a series of encounters with various numbers and types of creatures, plus one solo creature.
• Not all the encounters will necessitate a battle—the players will usually be free to choose how to deal with the monsters within the dungeon.
• Each dungeon will include one or two traps and/or hazards, separate from the monster encounters.
• Level-appropriate treasure, rolled entirely randomly, will be included.
• I will use any creature published by Wizards of the Coast and available on DnD Beyond. I will not include stat blocks—the DM will need, at the least, the Monster Manual. I will try to suggest alternate creatures from the MM in case the DM does not have access to the book in which the creature appears or access to DnD Beyond.
• I will provide very rough maps for some—but not necessarily all—of the adventures.

This week I present to you a dungeon for 4th-level characters, Dungeon #4: The Coral Reef.

The Coral Reef

• Location: The coral reef is located about a thousand yards off the shore on the coast of an ocean (or at least a very large sea). There should be some towns and/or villages nearby along the coast.
• Hook: Sahuagin have been raiding the towns and villages along the coast recently, and the townsfolk are getting desperate. A young girl rumored to have the gift of second sight has dreamed of something large and evil slumbering deep beneath the waves, and she believes the sahuagin are attempting to awaken it. Someone needs to go into their lair and destroy them.
• Complications: The townsfolk can have managed to trade with local traders for two potions of water breathing for each character. The girl’s dreams indicate that the magic in the coral reef causes steams of magical bubbles to flow up from crevasses at certain points in the caves within the reef, and inhaling these bubbles also work as potions of water breathing. The PCs will need to be careful, however, to ensure that they don’t get trapped under water without access to one of the potions.

Key to the Reef

Getting to the reef is the first problem. However, the girl has scrawled a crude map that will lead the characters between rock formations under the water to direct them to the specific cave entrance where the sahuagin have their lair. The PCs will need to swim underwater, so they’ll need to use one of their two potions to reach the reef.

Remember that, unless a character has an ability that grants a specific swim speed, their speed while swimming is half their normal movement speed. However, even characters with a 25-foot movement speed will still reach the reef within 30 minutes of entering the water.

Refer to the color map for the following area descriptions.

A. Cliff Drop

The PCs need to enter the water at a low (15’ above the water level) cliff. Unfortunately, a group of 10 giant crabs lair in many small tunnels that riddle the cliff side. When the walk down the sloped path the lower area where they should enter the water, the hungry crabs emerge from their tunnels and attack.

If the PCs leap into the water, the crabs follow. The crabs fight to the death.

B. Underwater Ambush

The PCs must swim straight ahead for 200 yards and go between two rock formations. When the PCs are between the rocks, a pair of giant octopuses emerge from crevasses on either side of the “path” and attack the PCs. These octopuses are trained by the sahuagin to attack any humanoids in the waters near the reef, and they fight to the death.

C. Shark!

When the PCs make the turn around the last rock formation and aim for the reef, any character with a passive Perception score of 12 or more sees the unmistakable silhouette of four sharks circling overhead.

The PCs have one round to react before the four reef sharks descend and attack. Like the octopuses above, the reef sharks are trained by the sahuagin to attack intruders. The sharks use their pack tactics ability to stay together and increase their chance of inflicting damage on the characters.

D. Sahuagin Cave

The entrance to the cave looks like any other crevasse in the miles-long coral reef. Only someone who knows where they are going, or who succeeds on a DC 20 Wisdom (Perception) check can find the cave while searching.

Note that the cave maps are designed to be stacked directly on top of each other, with Level 1 on the top, Level 2 in the middle, and Level 3 on the bottom.

Refer to the level 1 map for the following cave descriptions.

1. Cave Entrance

The entrance leads to a 20-foot long tunnel. The tunnel is dark and the walls, floor, and ceiling are made of coral.

2. Large Cave

This cave is 40 feet from floor to ceiling and entirely filled with water. The walls are made of rock, but bits of coral cling to the rock across nearly every surface. Some of the underwater fungus glows, giving off dim light.

In the southeast corner of the room, a crack in the floor lets up a constant stream of bubbles. If inhaled, these bubbles act as a standard potion of water breathing. Though the bubbles seem like air, they dissipate into the water about fifteen feet above the crack and disappear. The bubbles lose their magical property within 5 rounds if they are captured in a flask or other container.

The many passages from this cave are protected by a hunter shark and two reef sharks. These sharks attack any creature that is not a sahuagin or accompanied by a sahuagin. The sharks fight to the death, but do not pursue characters fleeing into the tunnels.

3. Dry Cave

The path between area 2 and this area slopes upward, and it appears a very large air bubble is trapped here. The air entirely fills this side cave section. There is enough air here for the PCs to spend up to 8 hours in this room. After that point, the air starts to become toxic.

The passage on the other side of the cave slopes back down to the same level as area 2.

4. Dead End

This side passage ends in a dead end. There is nothing of interest here.

5. Slithering Cave

All the passages leading into this cave slope upward, and the PCs climb out of the water as they reach the cave entrance. The rock walls glisten with moisture, and the cave floor is covered with dirt and large plants up to a height of almost 3 feet, potentially obscuring the vision of shorter characters. A few scattered boulders also provide additional cover for their cave’s inhabitants.

A giant constrictor snake along with four regular constrictor snakes live in this cave. The sahuagin feed them occasionally so that they only attack non-sahuagin.

The snakes slither through the underbrush until they get close to a character and then strike from ambush. The giant constrictor snake waits near one of the boulders and attacks when a character gets within 30 feet.

The constrictor snakes attack until they are at half hit points and then they retreat and attempt to hide among the underbrush. The giant constrictor snake fights until it is at one-quarter hit points and then retreats to a corner of the cave and fights only if pursued and attacked. The snakes do not pursue any fleeing characters who leave the cave.

6. Corrupt Cave

The coral and underwater plants in this cave look sickly and mutated. The cave entirely filled with water, is unlit, and it reaches 30 feet floor to ceiling.

A pair of sea spawn lair in this cave, along with four giant crabs. Like the sea spawn, the crabs are also crusted by barnacles and look mutated (no change to their combat stats).

The spawn and crabs attack (and pursue) any non-sahuagin and fight to the death.

7. Ahab’s Cave

This vast cave is mostly filled with water. The ceiling is 80 feet above the floor, and the top 20 feet is filled with air that constantly refreshes through the magic of the reef.

The cave is filled with all kinds of fish. Scattered among the various fish are a swarm of quippers. When the PCs move more than twenty feet into the cave, the quippers gather into a swarm and attack the closest PC. As soon as they attack, the killer whale in area 7a hears the frenzy and swims out to attack anyone invading its cave.

The whale is trapped in this cave—the sahuagin has some nefarious plan for it in the future—and while it is kept fed and has access to fresh air, it is enraged by any humanoids that come into its cave.

If the PCs have some way to communicate with the whale, they may be able to calm it down enough to get it to stop attacking. Unfortunately, it is too large to move through the tunnels, and so cannot easily be rescued from this cave.

8. Blood Frenzy Cave

This large cave is filled with water, and is dimly lit with the glowing underwater plants that can found in places throughout these caves. The ceiling is 40 feet above the floor of the cave.

A giant shark is kept here by the sahuagin, and it attacks any non-sahuagin who enters the cave. It is too large to pursue characters who flee into the tunnels.

9. Dry Cave

The path between area 2 and this area slopes upward, and it appears a very large air bubble is trapped here. The air entirely fills this side cave section. There is enough air here for the PCs to spend up to 8 hours in this room. After that point, the air starts to become toxic.

The passage on the other side of the cave slopes back down to the same level as area 2.

10. Living Water Cave

The passages from this cave eventually lead to the more important sahuagin leaders, and so they have placed a powerful guardian here. The cave is fully filled with water, and the roof is 25 feet above the floor.

A water elemental guards the passages leading further into the cave complex. It is completely undetectable in the water-filled room until it attacks. The elemental waits until the party fully enters the room, and then reveals itself as it moves forward and attacks whoever is in the front line of the party.

The elemental pursues fleeing characters and fights until destroyed, but does not attack any sahuagin, and does not attack if the party is escorted by sahuagin.

11. Magic Bubble Cave

This cave is completely filled with water, is unlit, and the ceiling is 20 feet above the floor. In the south corner of the room, a crack in the floor lets up a constant stream of bubbles. If inhaled, these bubbles act as a standard potion of water breathing. Though the bubbles seem like air, they dissipate into the water about fifteen feet above the crack and disappear. The bubbles lose their magical property within 5 rounds if they are captured in a flask or other container.

12. Sahuagin Shrine

This cave is 60 feet high, and filled with water up to a height of 40 feet. Ledges run along the west wall, from north to south, just above the surface of the water. The ledges are 5-10 feet wide at any given point.

A large altar sits on the floor of the cave, under the surface of the water. A sahuagin priestess resides here, praying before the shrine. A pair of reef sharks continuously circle the cave, protecting her while she performs her rites.

When any non-sahuagin enters the cave, the reef sharks immediately rush to the attack. On the following round, the priestess turns and joins combat. She cannot be bargained with, and fights to the death, as do the sharks, pursuing fleeing characters wherever they go (though the sharks cannot pursue outside of water-filled areas.

13. Poisoned Cave

The tunnels slope upward as one approaches this cave. The cave is 15 feet high, and water fills it to a height of 5 feet. Above the water, the walls are riddled with small holes.

The ground of this cave is covered with dirt, and underwater plants grow right up to the surface of the water, making it impossible to see more than a few feet in any direction.

A small stone altar is placed in the middle of the cave, and a sahuagin priestess stays here, conducting constant prayers to her god.

When the PCs enter the cave, the priestess hears their movement through the water unless they take precautions to move quietly. Two rounds after the PCs enter the cave, six poisonous snakes emerge from the holes in the wall and swim toward the characters. If the priestess is not already aware of them, the movement of the snakes alerts her.

When the snakes close with the PCs, the priestess rises from the water and attacks. She attempts to capture the PCs, if possible, though if they die from the snake poison, she doesn’t try to save them.

The priestess fights to the death.

14. Bubble Source

This is less a cave than a wide section of the tunnel. In the north side of the tunnel, a crack in the floor lets up a constant stream of bubbles. If inhaled, these bubbles act as a standard potion of water breathing. Though the bubbles seem like air, they dissipate into the water when they reach the ceiling and disappear. The bubbles lose their magical property within 5 rounds if they are captured in a flask or other container.

15. Open Cave

The ceiling of this cave is 60 feet above the floor, and the top 10 feet is filled with air. Glowing plants give off dim light and a large opening leads out of the cave complex to the water above.

A killer whale spends most of its time in this cave, in thrall to the sahuagin. A giant octopus hides among the rocks on the cave floor. When the PCs enter the cave, neither creature attacks right away, waiting until the PCs fully enter the cave and move toward the passage to area 21.

When the whale and octopus attack, they fight to the death. The octopus pursues fleeing characters, but the whale is too large to fit into the tunnels.

Refer to the level 2 map for the following cave descriptions.

16. Guard Post

This cave is entirely filled with water, and the ceiling is 15 feet above the floor. Four sahuagin are stationed here to prevent any intruders from moving further into the complex. The sahuagin immediately attack anyone entering the cave, and fight to the death. They pursue fleeing characters regardless of where they go.

17. Uneven Cave

This cave is 40 feet high, and filled with water up to a level of 20 feet. A series of ledges at various heights line the walls, most of them 5 feet deep, and a number of large boulders area scattered around the room, many of them reaching above the surface of the water.

A giant constrictor snake stretches out along the ledges, while a pair of sahuagin swim in the water beneath. The snake has learned not to attack the sahuagin, and so leaves them alone. However, it will attack and attempt to eat any non-sahuagin that enter the room. The sahuagin wait for the snake to attack, and then they join in battle, attacking whichever PCs are not grappled by the snake.

The snake fights until it is at one-quarter hit points, and then retreats up onto the ledges. The sahuagin fight to the death.

In the northeast corner of the room, a crack in the floor lets up a constant stream of bubbles. If inhaled, these bubbles act as a standard potion of water breathing. Though the bubbles seem like air, they dissipate into the water about fifteen feet above the crack and disappear. The bubbles lose their magical property within 5 rounds if they are captured in a flask or other container.

18. Guard Post

This cave is entirely filled with water, and the ceiling is 15 feet above the floor. Four sahuagin are stationed here to prevent any intruders from moving further into the complex. The sahuagin immediately attack anyone entering the cave, and fight to the death. They pursue fleeing characters regardless of where they go.

19. Lower Guard Post

The ceiling of this cave is 20 feet above the floor, and it is entirely filled with water. Two sahuagin and a merrow are stationed here. The sahuagin are terrified of the merrow, which is always hungry and previously killed and eaten a pair of sahuagin. The merrow serves the sea hag in area 26 below, and has been ordered to protect this area and leave the sahuagin alone, and it seems to be making the effort to do so.

The sahuagin and merrow attack any non-sahuagin entering the area and fight to the death.

In the northeast corner of the room, a crack in the floor lets up a constant stream of bubbles. If inhaled, these bubbles act as a standard potion of water breathing. Though the bubbles seem like air, they dissipate into the water about fifteen feet above the crack and disappear. The bubbles lose their magical property within 5 rounds if they are captured in a flask or other container.

20. Prison

The ceiling of this cave is 20 feet above the floor, and the cave is filled with water to about 10 feet. The air is continually refreshed through small cracks in walls as part of the magic of the reef, so characters can breathe in here indefinitely.

The three smaller caves on the north of this area rise up above the level of the water, and are blocked with heavy bars—requires a DC 15 Strength check to move enough for the prisoners to leave the cell. This is used as the prison for the sahuagin, where they keep captives until they are sacrificed to their god, or fed to one of the monsters that serves the sahuagin.

A pair of sahuagin are stationed in this cave to keep watch on the prisoners.

The northernmost cell holds a human merchant from the closest town on the coast. The merchant is terrified and wants nothing more than to get home again.

The cell beside him to the west holds a deep scion in human form. The deep scion has been placed here to “befriend” the merchant and the sahuagin plan to stage an escape for the pair, so that the merchant can vouch for the deep scion once they return to the town, and the deep scion can be a hero while secretly spying on the surface for the sahuagin.

When the PCs enter the cave, the sahuagin put up a fight, but surrender if reduced to half hit points. The deep scion recognizes the PCs as a real threat to the sahuagin and their plans. It stays in human form and asks to join the PCs—it tells them it is a warrior and only needs a weapon to help them fight the sahuagin.

During the next battle, the sahuagin shapechanges into its hybrid form and betrays the PCs.

Treasure: Each of the sahuagin carries a pouch with 2 potions of water breathing.

21. Shrine

This cave is entirely filled with water, and the ceiling is 25 feet above the floor. A large altar sits in the middle of the room, and a sahuagin priestess conducts her rites while a pair of sahuagin float nearby and guard her.

The sahuagin warn the priestess as soon as any PCs are spotted, and all three attack the party immediately. The sahuagin and the priestess attempt to capture the PCs (knocking them unconscious instead of killing them), but they do fight to the death.

Refer to the level 3 map for the following cave descriptions.

22. Heated Cave

The water becomes noticeably warmer as the PCs near this cave. The ceiling of this cave is 30 feet high, and the water is only 5 feet high. Cracks in the floor of the cave glow a hellish red, and the heat comes from these cracks. The two smaller caves on the east wall have floors above the level of the water. These areas are covered with thick vegetation. Steam fills the air above the level of the water throughout this entire cave area, reducing visibility to 10 feet.

A pair of hungry giant toads live in the two smaller caves on the east side. Three steam mephits float in the air in the thick steam, often arguing with each other and trying to boss each other around.

When the PCs swim through the underwater tunnel and enter the cave area, the toads wait in ambush, and the steam mephits stop arguing long enough to see what is going to happen.

If the PCs approach the smaller caves on the east side, the toads wait for the PCs to climb out of the water before attacking. If the PCs look like they swim through the cave toward the other tunnel, the toads leap out into the water and swim toward the PCs, attacking as soon as they are in range.

The steam mephits wait until the toads have swallowed a character each, and then they gleefully attack the remaining PCs so as to distract them while the toads swim away with their meals. The mephits fight until they are down to less than 10 hit points, and then they retreat and try to hide in the steam up above. The toads attack until they are down to less than 10 hit points, and they swim away and retreat to their individual cave areas. If pursued, the toads fight to the death.

23. Sauna Cave

This cave contains a giant air bubble that keeps out all the water. The ceiling is only 15 feet high in this cave, and the cracks in the floor give off heat like in the previous cave area. The room is filled with steam, limiting visibility to 10 feet.

A pair of sahuagin currently rest in this room, while four steam mephits fly about and argue and boss each other around.

When the sahuagin become aware of anyone entering the room—most likely hearing them before they become visible through the mist—they move to either side and ready themselves to charge out and ambush the PCs on the following round.

The steam mephits join in combat for the fun of it and to help even the odds against the sahuagin.

The sahuagin fight to the death, but the steam mephits retreat if they are knocked down to less than 10 hit points.

24. Kraken Temple

The tunnel, after descending to a great depth, curves upward again at the end, anyone entering this area must climb up out of the water at the entrance. The uneven stone walls of the tunnel end at the mouth of this chamber. The walls, floor, and ceiling of this chamber are made of giant stone blocks of a greenish, jade-like material (not jade and has no value). The ceiling is 30 feet high, and 5-foot-diameter pillars line the north and south walls. A huge altar occupies the west wall—a statue depicting a cluster of waving tentacles mounted atop it.

Standing in front of the altar is a kraken priest. He has allied himself with the sahuagin, and spends most of his time here venerating his god—a kraken that lives in an ocean far away from this coast.

The priest is automatically aware of anyone entering his temple. He moves behind one of the pillars and waits for the PCs to come within range of his spells before he launches his attack. He starts by casting evard’s black tentacles, and then uses his voice of the kraken to frighten anyone not affected by that spell.

The priest fights to the death and refuses any offers to surrender.

Treasure: A hidden panel in the base of the altar—requires a DC 15 Wisdom (Perception) check to notice—contains the kraken priest’s treasure. The PCs find a bone scroll tube with a scroll of green-flame blade, a scroll of toll the dead, and a scroll of control flames inside; one potion of healing; two 50 gp gems; four 250 gp art objects; and coins equaling 240 cp, 3,000 sp, 1,005 gp, and 42 pp.

25. Bodyguard Cave

This cave is entirely under water, and the ceiling is 20 feet high.

A merrow lairs here, protecting the sea hag in area 26 below. The merrow immediately attacks anyone entering this chamber, and fights to the death.

26. Hag’s Lair

This cave is entirely underwater. The ceiling is 15 feet high, and glowing underwater fungus provides dim light.

This is the lair of a sea hag. She is an ally of the sahuagin and, while she does not care about the kraken priest and his god, she is willing to go along with their plans as long as it causes destruction and despair to those who live on the surface world.

The hag always has a pair of sea spawn with her in her lair. The spawn serve the hag and help her with her rituals.

The sea hag is magically linked to the merrow in area 25 above. If the merrow dies, the sea hag is aware of it and prepares an ambush. She disguises herself as another of the sea spawn, and the three of them spread out in the room to make it difficult for area effect spells to hit all three of them at once.

When the PCs enter the room, the hag and the sea spawn swim toward the party, as if to attack. However, once she is within 30 feet of the rearmost party members (as she wants to affect as many PCs as possible), the hag reveals her true horrific appearance. She then tries to use her Death Glare on any frightened PCs, letting the sea spawn get between her and anyone trying to close to melee. She reverts to using her claws only if she has no other choice.

Treasure: The small cave on the south wall is the sea hag’s personal living quarters. A small, corroded metal box contains a potion of growth, a waterproof bag of dust of sneezing and choking, a philter of love, and assorted coins equaling 400 cp, 5,000 sp, 1,675 gp, and 70 pp.

27. Sahuagin Baron’s Lair

This large cave is entirely underwater. The ceiling is 30 feet high, and glowing underwater fungus provides dim light throughout the cave.

The sahuagin baron resides in this cave. He is the leader of all the sahuagin, and is allied with the kraken priest and the sea hag.

The baron generally floats in the middle of his cave in front of an altar to Sekolah, contemplating the mysteries of his god. He immediately attacks anyone who is not part of his force, and fights with every means at his disposal. He does not surrender, nor does he take prisoners.

Treasure: The baron has a large wooden chest. The chest is locked, and the baron keeps the key on a small cord around his neck. The chest contains medium-size mithral scale mail armor, a potion of climbing, a potion of healing, three 50 gp gems, two 250 gp art objects, and assorted coins equaling 960 cp, 12,000 sp, 4,020 gp, and 168 pp.

Maps

This is the area map for the path the PCs must take to reach the reef.

This is the map of Level 1.

This is the map of Level 2.

This is the map of Level 3.

Update and Conclusion

Like last week, this was a much larger dungeon than the first couple. Providing enough experience points to advance a party of five characters a full level becomes a greater challenge each time they go up a level.

Which brings me to a change I’m going to make to the Dungeon-A-Week format starting next week. This blog is a secondary thing for me—it takes a backseat to the work I do on my novels. I’m finding that the blog is taking too much time away from my other writing, and that’s not something I want to let happen.

So, I’m going to continue the Dungeon-A-Week project, but I’m going to put a time limit on myself each week. That means that, as the dungeons go up in level, I’m going to start spreading them across more than one week.

I intend to provide a complete “section” of a dungeon each week, so that it’s usable pretty much right away. It might be the first bunch of rooms, or the first “level” of the dungeon, or so forth. And I’ll try not to spread a particular dungeon over more than 2 or perhaps 3 weeks, as I will continue to keep them simple.

Again, I hope this material is of use to you. If you end up using it, either whole or in parts, please drop a comment and let us know how it went.