Long Island Dungeon Master · A Creature Keeper

The Undertow

It doesn't want to kill you. It wants to keep you.

It doesn't fight on the surface. It never has to. It waits below, still as the riverbed, until someone wades too close or leans too far over the water, and then a hand closes around an ankle and the fight isn't a fight anymore. It's a rescue, and the clock is a breath.

This is an old troll's trick worn down to its cruelest shape: it doesn't need to win a fair fight, because it never offers one. It just needs the water, and the water is always willing to wait.

This is a free monster: everything on this page is table-ready, and the whole file loads into Gnasher's Grimoire as a Creature Keeper, tables and all.

Field Notes

how to run a fight that's really a rescue

Gnasher's Take

A grapple is a speed bump at most tables: fail the check, waste a turn, move on. Don't let this one be that. The moment it drags someone under, the fight flips. Everyone still on the bank is running a rescue now, against a breath they can't see. Let the drowning rules do the work; you don't need anything fancier than what's already in the book.

How to run it

The plain-English version: an ambush predator that wins by making the party choose between fighting it and saving their friend.

Flip for the five-beat breakdowntap to flip

The wait. It's already there, submerged and still, before anyone arrives. Roll on Signs Something's Below as the party gets close to the water.
The grab. It opens with Drag Under on whoever's nearest the water's edge, not the strongest target: proximity to the water is the only targeting logic it has.
The split. The grabbed PC is underwater, restrained, and drowning by the book's own rules. The rest of the party has to choose: fight what they can reach, or work the grapple to pull their friend free.
It doesn't chase far. It wants to keep the fight in the water, where it regenerates fast. Drag it onto dry land and it stops healing entirely; that's the real answer, not more damage.
It disengages below about a quarter of its hit points, or immediately if it ends up beached, sinking back toward the deep water and waiting for the next traveler.

The win condition isn't just the kill. Getting it out of its water is. A party that hauls it onto the bank has out-thought the encounter with a rope and some nerve.

The Tables

roll the drowning

Who's Already Been Taken (d8)
1	A ferryman who won't cross after dusk, no matter the pay.
2	A fisherman's net comes up shredded, and heavier than it should be.
3	A child who wandered too close to the bank and was pulled under, but came back days later, gasping, changed.
4	Livestock found drowned in a field nowhere near the river.
5	A rope bridge with claw marks scoring the underside of the planks.
6	A shrine to "the toll" where offerings are still left, though nobody remembers why.
7	A boat found empty, mid-current, oars still in the locks.
8	Someone swears the river ran backward for a moment, right before the screaming started.

Signs Something's Below (escalating) (d10)
1	The water goes glass-still for a moment, even in the current.
2	Fish leap onto the bank, fleeing something below.
3	A cold patch in the water, out of season, that doesn't move with the current.
4	Bubbles rise in a slow, deliberate line, toward the shore.
5	Something brushes a leg underwater, gone before anyone can react.
6	The riverbank mud shows a drag mark, one direction only, into the water.
7	A held breath, not your own, echoes up from beneath the surface.
8	The current pulls harder than it should, toward the deep water.
9	A shape, too large, passes beneath a boat and doesn't surface.
10	Something grips an ankle from below, and does not let go.

Where the Water Waits (d6)
1	A toll-bridge everyone still pays to cross, out of superstition nobody admits to.
2	A fishing village that's quietly stopped fishing the deep channel.
3	A flooded quarry that used to be a riverbed.
4	The wreck of a ferry, still visible at low water.
5	A millrace where the wheel hasn't turned in a generation.
6	A tide-pool cave that floods completely at high water.

Who it’s for. The Undertow is a solo CR 5 encounter (1,800 XP): a medium fight for four 5th-level PCs if it ends up fought on dry land, where it can’t regenerate and loses its main trick, but deadly if the fight stays in deep water, where regeneration and Drag Under both favor it hard. The terrain is the real difficulty dial here, not the raw CR: say so out loud if you’re running it for a table that doesn’t know the trick yet.

The Stat Block

one whole monster, free, forever

An original creature, compatible with fifth edition. Drop this file onto the Grimoire and the Undertow walks onto the War Table: its attacks roll themselves, and all three tables load into the Summon.

The Undertow

Huge giant (troll), chaotic evil

Armor Class 15 (submerged, hard to track)Hit Points 93 (11d10 + 33)Speed 10 ft., swim 40 ft.

STR18 (+4)

DEX10 (+0)

CON18 (+4)

INT7 (-2)

WIS12 (+1)

CHA9 (-1)

Skills Stealth +6 (advantage while submerged), Perception +4

Senses darkvision 60 ft., passive Perception 14

Languages Giant, understands but rarely speaks

CR 5 (1,800 XP) · Proficiency Bonus +3

Bound to the Water. While submerged in its home water, the Undertow regains 10 hit points at the start of its turn if it has at least 1 hit point. If it starts its turn entirely out of the water, this trait doesn't function, and it doesn't regain hit points lost this way until it's fully submerged again.

Hidden Below. While submerged, the Undertow has advantage on Stealth checks, and it's heavily obscured to any creature that isn't also underwater with it.

Actions

Multiattack. The Undertow makes two Claw attacks.

Claw. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 17 (3d8 + 4) slashing damage.

Drag Under. Melee Weapon Attack: +7 to hit, reach 10 ft., one target. Hit: 17 (3d6 + 4) bludgeoning damage, and the target is grappled (escape DC 15) and restrained. The Undertow can only grapple one creature at a time this way. If it's in deep water, it can then swim up to half its speed without provoking opportunity attacks, taking the grappled target below the surface with it.

Gnasher's Notes

A grappler is the easiest monster in the world to run badly, because most tables treat a grapple like a speed bump: fail the check, waste a turn, move on. Don't. The whole horror of the Undertow is that the fight isn't against the monster, it's against the water, and the water is patient. The instant it drags someone under, the table's job flips: everyone still on the bank is running a rescue now, not a battle, counting rounds against a breath they can't see. Let the drowning rules do the work; you don't need anything fancier than what's already on the book. And give your players the actual answer, because it's a good one: this thing regenerates like a nightmare in its own water and doesn't regenerate at all on dry land. A party that figures out to haul it onto the bank has out-thought a CR budget with a rope and some nerve, and that's the best kind of win. It doesn't chase far past the waterline; it's old, it's patient, and there will be another traveler along the ford before long. Stay hungry, Gnasher.

Coll Ferry-Ward, who pulled her brother out

Role: a young waterman who dragged her brother from the depths half-drowned.
Wants: to stop anyone else from going in alone.
Voice: counts everyone twice before a crossing, out loud, without meaning to.
The line: “It doesn’t want to kill you. It wants to keep you. There’s a difference, and it’s worse.”

Stay hungry. — Gnasher

The Undertow is an original creature and fully compatible with fifth edition. This file is a Creature Keeper: open it anywhere (it works offline), or load it into Gnasher’s Grimoire — the free DM app at longislanddungeonmaster.com/grimoire — and its monster and tables come along with it.

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This work includes material from the System Reference Document 5.1 (“SRD 5.1”) by Wizards of the Coast LLC, available at dnd.wizards.com/resources/systems-reference-document. The SRD 5.1 is licensed under the Creative Commons Attribution 4.0 International License, available at creativecommons.org/licenses/by/4.0/legalcode.

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