No Prep Module: The Hydra's Flood (for D&D or RPGs)

The Merfolk Resort

This adventure can be can be easily customized to a party of any size or level, or run as a level 5 adventure for 5 players. It can work as a small side adventure in an existing campaign, as a standalone one-shot, or the start of a new adventure. Use the [Customize] links to the tools in their relevant section to customize the adventure to your party.

Start with a week of downtime and choose something from each PC's backstory to advance and connect to the adventure. A relative, contact, or rival could be involved, or something pertaining to one of their character's goals can be progressed by something in the adventure. If a PC has no hooks in their backstory, use the downtime to establish what one of these might be instead: have someone ask about a relative, introduce a rival, or ask about their character's goals. If this is the first (or only) session with these characters, you can skip this.

You can also generate different descriptions using the Description Generator


You are at a seaside resort, a picturesque paradise surrounded by crystal-clear waters and a lush tropical landscape. The resort is operated by merfolk, who welcome visitors with warm hospitality and offer them a glimpse into the underwater world. The merfolk have built stunning structures that are both beautiful and functional, including spacious guest rooms, lively dining areas, and relaxing lounges.

Explore: if the PCs explore the resort, go to #2

Quest: if the PCs try to strike up a conversation or find a quest, go to #3


As you wander through the resort, you come across a hill that overlooks the ocean. This hill is known to be the site of a great battle that took place long ago. Despite the violence that once occurred here, the hill is now covered in a blanket of vibrant wildflowers. Visitors often leave offerings of flowers at the base of the hill, paying homage to those who lost their lives in the battle.

Event. There's a merfolk lottery with a magic item as the reward where the PCs can buy as many raffle tickets as they want. If any of the PCs spend all (or almost all) of their money, or if collectively as a group they spend more than the group's average wealth, then they'll win a random magic item. [Customize]

Quest: Regardless of what the PCs do next, they'll be approached by a tortle at #3


You see a large tortle, standing at 6 feet tall, who carries herself with a regal grace. Her black surcoat is adorned with intricate embroidery and she wears a tan shirt underneath. Tiny earrings sparkle along her head and a worn animal skin hat sits atop her head, dotted with cuts and holes. She introduces herself in a deep, resonant voice. "Greetings, travelers," she says, bowing her head in respect. "I am Lim, and I guess you could say I'm something of a traveling historian."

Personality: Lim is patient, understanding, and willing to share her wisdom with others. She's also a bit whimsical and unpredictable, with a love for adventure and a thirst for knowledge. She has a strong sense of empathy and a desire to see those in need get the help they need, even if she lacks the physical ability to do so.

After some introductions, or if the PCs ask about how things are at the resort, or any rumors, or quests, she'll say the following:

"The flooded mountains have become a constant source of trouble for everyone here at the resort. The portals to the elemental plane of water have allowed all manner of dangerous creatures to come through and threaten the merfolk's peaceful way of life. We've seen creatures we've never seen before, and they're not always friendly. It's a treacherous place, and the resort is not well-equipped to keep everyone safe."

    Lim also knows the following:
  • The merfolk and their customs, and can help any PCs navigate any requests they may have about the resort
  • The flooded mountains nearby and can clue the PCs about an alternate way into the lair besides the treacherous path
  • Lore about the portals and plane of water, and can give a clue about the puzzle in #9

Leave: When the PCs decide to leave and check out these mountains, you can continue to #4


Tall, jagged peaks rise up in every direction, casting long shadows over the landscape. The ground is covered in moss and lichen, and there are a few gnarled trees and shrubs scattered throughout the area. The strong winds blowing through the mountains carry a chill, and you can see your breath in the air.

Encounter. There's 1 Sahuagin Priestess (CR 2: Monster Manual p. 264), 1 Fathomer (CR 2: Princes of the Apocalypse p. 207) (Easy, 180 XP each) [Customize] here.

Search: If the PCs decide to search the area, a DC 17 Investigation check (DC 12 if they received a clue from Lim) will find an old tunnel that leads to #7

Path: there's a narrow path approaching the entrance to a cave at #5


A narrow and treacherous path leads up to the entrance of a cave, with steep cliffs on either side. The area is filled with the sound of rushing water, and you can see streams and waterfalls cascading down the sides of the mountain. The air is thick with mist, and the visibility is low. The only sounds are the crashing of water and the occasional screech of a bird or the growl of a wild animal.

Trap. Gusts of water and low visibility threaten to push PCs off the narrow ledge.

Effect: Sight is heavily obscured and any PC that crosses needs to make a DC 15 Acrobatics check or Dexterity save. On fail they fall off and take 26 (4d10) bludgeoning damage.

Countermeasures: Any effect that allows teleportation or can otherwise bypass the ledge can allow a PC to pass safely

Crossing: the ledge leads to #6

Return: to the previous area leads to #4


Enemy Patrol: 1 Water Elemental Myrmidon (CR 7: Mordenkainen's Tome of Foes p. 203), 1 Kuo-toa Whip (CR 1: Monster Manual p. 200) (Hard, 620 XP each) [Customize]

Chance of Encounter Per Hour: 35%

As you make your way up the rugged mountain path, you soon find yourself standing at the entrance of a dark and foreboding lair. The air is thick with the sound of rushing water, and the ground beneath your feet is slick and slippery. You can see that the area has been flooded, and the water level is rapidly rising.

Treasure guarded by a cowardly non-hostile monster that demands the PCs leave (Intimidation DC 12, any other solution DC 17). On success, the monster flees and they get 800 cp, 3000 sp, 2300 gp [Customize]. On fail, a Sahuagin Baron attacks with surprise.

West: leads to #5

South: leads to #7


You find yourselves in a large chamber filled with towering pillars of rock and glowing crystal formations. The walls of the lair are slick with moisture, and streams of water flow down from the ceiling, forming small pools on the ground. In the center of the room, you see a massive portal, shimmering with a blue-green light. The water pouring into the lair is coming from this portal, and the sheer force of the water rushing through the portal is both awe-inspiring and concerning.

Encounter. There's 1 Water Elemental (CR 5: Monster Manual p. 125), 1 Bullywug Croaker (CR 2: Ghosts of Saltmarsh), 1 Sea Hag (CR 2: Monster Manual p. 179) (Hard, 540 XP each) [Customize] here.

West: there's a secret tunnel (Investigation DC 12) that leads out to #4

East: leads to #9

North: leads to #6

South: leads to #8


You come across several small elemental shrines dedicated to the forces of water. The shrines are simple yet elegant, made from stone and shells that have been collected from the surrounding area. They're situated in quiet, out-of-the-way alcoves, surrounded by offerings of fresh flowers and polished stones.

Event. 1) Fight a Deadly encounter to receive a treasure hoard, or 2) upgrade a magic item but lose all hit dice and healing spell slots, or 3) heal to full HP but become cursed with 24h Vulnerable. Offered by elemental spirits within each shrine, but you can only pick 1. [Customize]

East: leads to #10

North: leads to #7


You come to a chamber with crystal formations glowing with a blue-green light. They tower over you, reaching towards the high ceiling of the chamber. The crystals are transparent, yet they seem to contain a swirling, aquatic energy within them. They are etched with intricate patterns and symbols, and they emit a soft, pulsing light that illuminates the chamber. The formations are made up of thousands of smaller crystals that have grown together over time. The crystals seem to be reacting to the water pouring into the lair, pulsing in time with the flow of the water.

Key. There's a Puzzle [Customize]: 7 of the crystals can be moved and placed on a 360 degree circle, and are each a different color: blue, yellow, orange, indigo, green, violet, and red. To correctly solve the puzzle they must be placed in the right color order: ROYGBIV and aligned to match the flow of water in the room. To make the puzzle easier, you can forego one of those requirements. To mark progress, there can be two large dull crystals above the puzzle, and when one of the steps is correct then one of the crystals lights up. Solving the puzzle shuts off the flow of water at the green path at #12.

    If the PCs further examine the room or get stuck, here are some additional clues:
  • Carvings on the walls that depict the flow of water.
  • The water flows on the walls are colored in a spectrum.
  • The carvings end at rainbow waterfalls, with arrows showing the flow of water.

West: leads to #7

South: leads to #10


The water is murky and cold, and it rises up to your waist as you wade through the tunnel. The walls are slick and covered in algae, and the only light comes from the dim glow of the crystal formations that line the walls. The current is strong, and you can feel it pulling at your legs as you struggle to keep your balance.

Trap. Attempting to go further into the room causes a huge wave to sweep away PCs.

Effect: DC 11 Constitution save or take 7 (2d10) bludgeoning damage and gain a level of exhaustion.

West: leads to #8

North: leads to #9


You come to a deep chamber with crystal clear water, and you can see vibrant coral formations with schools of colorful fish swimming between the underwater plants. The garden is lit by glowing crystal formations, and you can see that the plants are thriving in the warm, mineral-rich water. Delicate sea flowers sway in the gentle currents, and you can hear the soothing sounds of water lapping against the coral. Schools of small, brightly colored fish swim around you, and the garden is home to a variety of sea creatures, from graceful seahorses to tiny crabs and shrimp.

Encounter. There's 1 Sahuagin Champion (CR 3: Ghosts of Saltmarsh), 1 One-Eyed Shiver (CR 3: Princes of the Apocalypse p. 207), 1 Crushing Wave Reaver (CR 1/2: Princes of the Apocalypse p. 205), 1 Sahuagin High Priest, Cybele (CR 5: Ghosts of Saltmarsh, p.251) (Deadly, 660 XP each) [Customize] here.

East: is an unlocked door leads to #3

North: leads to #12


You come across a magnificent underwater grotto, framed by towering columns of coral, and filled with a dazzling array of marine life. There's giant clams with iridescent shells, as well as vibrant sea anemones and delicate sea flowers in this underwater paradise.

Event. Receive 2d10 (4) healing but become Prone for 24h, otherwise fight a Medium encounter. Offered by exotic fruits found underwater, or by attempting to plunder the clams and coral. [Customize]

North: (green) is a torrential geyser that continuously pumps powerful water blasts into the corridor that deal 2d10 bludgeoning per round leads to #13

South: leads to #11


The air is thick with the scent of seaweed and salt, and you can hear the sound of water echoing through this large chamber. As you make your way inside, you see that it's filled with glittering crystal formations that pulse with a deep blue light. In the center of the chamber, you see a massive nest of seaweed and coral.

Encounter. There's 1 Sahuagin Priestess (CR 2: Monster Manual p. 264), 1 Hydra (CR 8: Monster Manual p. 190) (Deadly, 870 XP each) [Customize] here.

Treasure. Within the monster's hoard is 900 cp, 6000 sp, 2300 gp, Dust of sneezing and choking, Alchemy jug, Rope of climbing, Elemental gem


The merfolk are overjoyed to learn the monsters have been cleared out and welcome the PCs as heroes. They shower them with crafted gifts and food as a token of their gratitude, and Lim is especially appreciative of the PCs' bravery. She invites the PCs to stay at the resort for as long as they like, offering them a chance to relax and enjoy the sun, sea, and sand.

This could lead into further adventures where the PCs actually head into the elemental plane of water to find further adventure or perhaps seek a way to close the portals for good.

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