Your Players have taken a break in a small coastal village as they make their way from one adventure to another. It’s a lovely place, with beautiful beaches, quaint local festivals, and some of the best seafood the dice can roll up.

This place is prosperous, beautiful, and happy.

Maybe too happy.

The festivals stop at sundown. Before the light vanishes from the sky, the children are brought indoors. The village’s strongest and best position themselves with spears, nets, and tridents, all posted on the beach. Facing the water. This is a nightly ritual, performed with the precision only years of repetition can create.

When your Players ask why the villagers are doing this, they only smile and say: “One night, the merfolk will come. And then we will be ready.”

What a wonderful way to introduce the fact that this village was built on reclaim land, taken from the clutches of the Merfolk long ago. Or perhaps their prosperity is directly attributed to a Merfolk relic that they keep safe and sound in their leader’s hut. Or maybe they have a ritual sacrifice of a Merfolk every decade or so that ensures the continued survival of their village.

However it happened, this village has made the Merfolk very angry. And that is never a good idea.

When you put Merfolk in your adventure, you’re essentially introducing an alien civilization. These are people who thrive in an environment that would kill your players. Even the fiercest Paladin on land will find that they’re not an apex predator anymore.

They’re a drowning animal with a sword.

The world of the Merfolk should be something that your players can’t just loot with a sword and some fireballs as though it were an ancient temple on land. You can lean into the utter strangeness of a Merfolk world – strange physics, complex social structures, the thick silence and unbroken darkness of the deep sea. Get them involved in customs and behaviors that make the most Byzantine court on land look like a children’s playground. They’ll need to think with more than their weapons if they want to survive the politics of the deep.

Once they do, though, the Monster Manual gives you a little variety to play with. Of course you’ll have your Merfolk Commoners and Nobles and the like. But there are also Merfolk Skirmishers and Merfolk Wavebenders – the martial and magical defenders of this kingdom. You should consider this distinction carefully, and ask how you can use them to create conflicts that will pull your Players in.

Skirmishers are adaptable, direct, physical; Wavebenders are mystical, strategic, slow to act.

Are these different Orders? Castes? Clades? The players might meet one faction first, only to realize later they’ve picked the wrong side in a war they don’t understand.

And, of course, when that war begins, pulling your Players in whether they want it or not, the combat that you put in front of them will be unlike any other combat they face on land. The three-dimensional encounters, changing currents, and movement-sapping weapons can seriously hinder players who are already largely at a disadvantage by fighting underwater. When the Wavebenders get involved, your Players might feel them before they see them, as tides shift and bend around them.

And if you want to be really nasty, give your Wavebenders a few uses of Dispel Magic.

That’s a real nice Waterbreathing spell your Wizard put on you all. It’d be a shame if something… happened to it.

In that fight, you can pull in nearly any aquatic monster you want from the Monster Manual – Sharks, Giant Squids, Marids, Merrow, Water Elementals are all players in this fight, if you want them to be.

And then, within that strange, alien war, your Players discover that the Merfolk don’t serve the ocean. They serve That Which Sleeps Beneath. A sleeping tidal god. A distributed coral reef consciousness. An ancient AI that thinks in sonar and phosphorescence.

In the midst of pitched battle, one of the Wavebenders goes stiff and silent. Their mouth opens as blood leaks into the water from their eyes and ears, bringing the Sharks to them. Their grim expression, rigid limbs, tight throat tells your Players that the Wavebender did not call what is coming. They were chosen.

In a terrible, wailing voice, pitched to resonate like whalesong and travel thousands of miles at a time, the Wavebender intones: “I COME.”

And now it’s not a factional spat in a Merfolk kingdom.

Now it’s a desperate battle for the Seas themselves. And your Players are way out of their league.

That’ll teach them to think they can have a beach episode and get away with it.