The Arm Through the Wall
The hall slopes down into black water. Halfway along the north wall the masonry has failed — not fallen, failed, pushed inward in a long vertical crack the width of a man — and out of that crack comes an arm.
It is forty feet of it and there is more behind. It lies across the flooded floor the way a rope lies on a deck, moving very slightly, and where the suckers have gripped the stone they have taken the stone with them. Somewhere beyond the wall, something enormous shifts its weight, and the water in the hall rises an inch and does not go back down.
DM summary. One limb of a thing far too big for the room, reaching in through a crack that is getting wider. You cannot kill what is on the other side of the wall. You can take its arm off and be somewhere else when it notices.
The Boon
Sostratos the BailerOptional companion
A sponge-diver on the sluice wheel who calls the water a round before it moves
What it is. A short, wide man of about fifty with salt-split hands, a cork float on a cord round his neck, and a bronze bailing scoop he has not put down. Sostratos the Bailer dived sponges as a boy and bailed a tribute hull for eleven years, so he has spent his life in water that was trying to get in. He is not brave and does not pretend to be.
Where it comes from. Any early room with water or a store-hole: a hoarse voice behind a fallen grate (Athletics DC 13 to lift, or a crowbar and no check), a man asleep on a resupply room's sacks, an NPC's cousin. He asks for water first, then follows the light. He does not volunteer that he knows drains. Late plant: the drain-mouth one room back, or this hall on round one, up the second column, water at his knees.
What it does here. He knows the wheel: halls like this drain the same way, and the pawl comes off the ratchet before the bronze turns, so the sluice opens in two rounds of Athletics DC 14 instead of three. Keep the two effects apart. The wheel is cheaper, which is a whole round of somebody's action given back. And he will grind a round on it himself at +3 while the party works the wall — that round drops the water exactly like any other, and it does not tick the four-success track, because the track is only ever things the players did. He reads the water too, and calls the crack tearing wider one round before Phase 2 — the only warning this room gives. He will also hold the stair door while they cut.
What it doesn't do. He will not go below the flood line or within 30 ft of the crack, and says so. He does not fight, will not touch the sinew, cannot break a grapple. Abandoned, he does not drown — six minutes of air, and he knows the bore — unless a Drowned Hoplite still stands, in which case they have him. Without him the wheel is three rounds and the hall is exactly as written.
Card and numbers or jump to the card ↓
Arena
Battle map: The Undercroft Breach — pages 19 & 20, 110 × 75 ft
The undercroft breach is the room: the printed cave-in is the crack in the north wall the arm comes through, and the blue-tiled chamber reads as the flooded low end. Chalk the six squat columns in two rows, shade the water line, and move it a foot up the slope each round.
- Dimensions. 80 ft × 60 ft, ceiling 25 ft, and the floor slopes 6 ft from the south doorway down to the flooded north end. The party enters dry at the high end and will not stay that way.
- Light. Whatever they brought. The water throws it back at them in pieces.
- The water. Two feet deep at the north wall when the party arrives, and rising 1 ft per round as the crack widens. Water 3 ft or deeper is difficult terrain; at 5 ft the north half is swimming, and swimming creatures make attacks with disadvantage unless they have a swim speed.
- The crack. A vertical failure in the north wall, 2 ft wide at the start and 6 ft wide by round 5. The arm comes through here. So does the water. So, if the party is very unlucky and very slow, does a second arm — see Phases.
- The columns. Six squat columns in two rows down the hall, 4 ft thick, half cover. The arm uses these: it wraps a column and pulls, and a pulled column comes down in a 10-ft line (DC 15 Dexterity save, 22 (4d10) bludgeoning, and the rubble is difficult terrain and a place to stand above the water).
- The sluice wheel. A bronze wheel on the west wall at the high end, connected to a drain under the floor. Three rounds of Athletics DC 14, one PC per round, opens it; open, it drops the water level 2 ft per round until the flood outpaces it again on round 8.
- The stair. A dry stair in the south-east corner climbing 10 ft to the exit, above the flood line. The arm can reach the bottom three steps and no further.
Roster & Budget
One limb, and the dead it has been washing in here for a month.
| Count | Creature | XP each |
|---|---|---|
| 1 | The Arm Through the Wall (boss) | 7,200 |
| 3 | Drowned Hoplite | 700 |
Base 9,300 XP; ×2 for four bodies = 18,600 adjusted, against the APL 9 Deadly budget of 14,400. That is 1.3× over on purpose, and the excess is honest in two directions: the arm's 189 hit points are a severing threshold, not a kill, so a party that pays full price for them gets no corpse; and the clock means most tables will never fight all four at once. Two hoplites are standing in the water when the party arrives. The third washes in on round 3.
The Kraken Spawn on the other side of the wall is not on this table and is not statted here. It is scenery with an appetite. If your players insist on going through the crack to find it, see DM Notes, and be honest with them first.
Phases / Clock
| Phase | Trigger | What visibly changes |
|---|---|---|
| 1 — Reaching | Start | Reach 30 ft. The arm sweeps, grabs, and drags toward the crack. Water +1 ft per round |
| 2 — Shouldering In | The arm drops to half HP, or the start of round 5 | The crack tears wider with a noise like a ship breaking. Another fifteen feet of limb comes through: reach 40 ft, Ink and Surge recharges, and the arm can hold two grappled creatures instead of one. Water +2 ft per round |
| 3 — It Notices | Round 9, if the fight is still going | Something on the far side pulls. The arm goes rigid, then retracts hard, taking everything it is holding through a two-foot crack. Do not bluff this. Tell the party on round 8 that the arm has gone still, and let them decide |
Round one. The arm does not attack. It gropes along the floor, finds one of its own Drowned Hoplites, closes on it, and takes it back through the crack without hurrying. The wet crunch is the whole scene: the party has just watched what a grapple means here.
Combat Resolution
Fighting means severing, and severing means getting inside its reach.
- Tactics. The arm fights the room, not the party. It pulls columns down on clustered PCs, drags grappled creatures 20 ft toward the crack per turn, and ignores anything on the dry stair. It cannot be flanked, feared, or charmed, and it has no idea what a spell is.
- The Drowned Hoplites are slow on the floor and fast in the water. They work the flood line, dragging anyone who falls prone into the deep end, and they do not pursue up the stair.
- The weakness. Where the crack pinches the limb there is a ribbon of white sinew, a hand's width across, visible to passive Perception 15 or Investigation DC 14 once anyone gets within 20 ft of the wall. It is AC 20, and 50 slashing damage in a single round severs the arm outright regardless of its remaining hit points. Cold damage to the sinew halves the required total for that round; fire does nothing at all through the water.
- What winning looks like. The arm comes off, or it hits 0 hit points, which amounts to the same thing: the limb thrashes, floods the hall to chest height in one surge (DC 16 Strength save or be swept 20 ft and knocked prone), and withdraws the stump. Forty feet of dead tentacle stay behind, which is worth something to a cook and a great deal to an alchemist. Five to seven rounds, 30 minutes.
- What losing looks like. A PC dragged through the crack is gone from this scene, not necessarily dead. They surface in a sea cave two rooms of fiction away, bruised, at 1 hit point, and can be recovered in the Resolution — which is a far better beat than a body.
Alternate Resolution
Skill: hold the wall, or empty the room. Four successes before three failures. Each attempt is one action by one PC. Announce the track out loud.
| Approach | Check | On a failure |
|---|---|---|
| Drive an iron pin into the crack above the limb | Athletics DC 16, or Thieves' Tools DC 14 with a crowbar or a spear-butt | The pin skips; 11 (2d10) bludgeoning to the pinner and the arm gets a free grapple attempt |
| Read the masonry and find the course that is holding | Investigation DC 15 or Stonecunning / mason's tools, no check | The party braces the wrong course; water +1 extra foot this round |
| Open the sluice | Athletics DC 14, three rounds as above; each round counts as one success | The wheel slips; the wheelman takes 7 (2d6) bludgeoning and loses the round |
| Salt and cold in the water | Any cold spell of 2nd level or higher, or two flasks of brine from a Deep Water Kin room | No failure state; this one is a gift for a prepared party and it can only be used once |
| Sing it back | Performance DC 18, only if a Siren of the Sunken Court has been heard earlier in the run and someone paid attention | The arm strikes at the singer specifically, with advantage |
If the party brought Sostratos the Bailer, he kicks the pawl off the ratchet before anybody touches the bronze, and Open the sluice takes two rounds instead of three — for whoever turns it. He will grind out one of those rounds himself at Athletics +3 while the party works the wall, and the water falls for it like any other round on the wheel, but it does not count on the track. Four successes are four things the players did.
- Four successes: the crack is pinned and the water is falling. The arm can no longer widen its way in, withdraws to reach 15 ft, and after two more rounds it gives up and leaves. The party has won without killing anything, which is the intended shape of this finale.
- Three failures: the north wall goes. The hall floods to 10 ft in two rounds and the exit is the stair, at a swim, with the arm at full reach behind them. Nobody dies for this automatically; everybody loses a round of gear.
- Sever-and-flee counts as a win. A party that cuts the sinew and runs for the stair has resolved this finale properly and should be told so. There is no bonus for standing in the water to watch.
Scaling — APL 6 / 12 / 15
| APL | Adjustment |
|---|---|
| 6 | Arm at −25% HP (142) and reach 20 ft. Two Drowned Hoplites, none arriving later. Water rises 1 ft every other round. All DCs −2 |
| 9 | As written |
| 12 | Arm at +25% HP (236) and a third legendary action. Four Drowned Hoplites plus one Siren of the Sunken Court on the far ledge. Water +2 ft per round from the start. DCs +1 |
| 15 | Two arms, entering by separate cracks in the north and east walls, each at the written statistics with 150 hit points. A Scyllan Horror works the deep end. The severing threshold rises to 65 damage per arm, and Phase 3 arrives on round 7. DCs +3 |
Exit to the Resolution
- Pacing. 30–40 minutes. Behind schedule: start the water at 4 ft, skip the Drowned Hoplites entirely, and run it as the arm and the clock — 20 minutes, and it loses nothing.
- Do not roll the water. Say the number every round: "three feet," "four feet," "five, and the columns are wet to the collar." A clock the players hear counted is a clock they play against.
- If someone goes through the crack, be honest before they commit: the far side is open black water and a Kraken Spawn's home range, there is no exit that way, and a PC who swims through is choosing to die in the dark unless the party hauls them back within one round. Say it in your own voice, out of character. Then respect the choice if they make it.
- Table safety. Drowning, submersion, and being dragged under. The soft framing: the arm drags people toward the crack and pins them against the wall rather than under the surface, and nobody is ever described as breathing water. Offer that framing before the first grapple, not after.
- Clue drip. This finale delivers the Path tier, and two objects here carry it well: the water itself, which is salt and tidal and rising on a schedule that anyone can time (X16 The Salt Door, X11 The Undercroft River), and the drowned kit on the hoplites — Knossian bronze from a wreck outside the maze, which means outside is closer than anyone thought (X05 Carry Them Out if the party is looking for someone).
- If Sostratos is left behind, say it once and let it stand. He is not dead unless a Drowned Hoplite reached him — he has the air and he knows the bore — but he is finished with the party either way, and a man who thanked them in the second room and was not on the stair when the door shut is the loudest thing in the Resolution. Do not have him turn up cheerful.
- Callback. A party that crossed the black pool in R036 or the sunken peristyle in R098 has met the Deep Water Kin's habit of grabbing and dragging. This is the same tactic at four times the size, and a player who says "it's going to pull us in" out loud has earned being right.
Boon Card
Sostratos the Bailer
Medium humanoid, neutral · CR 1/8 (25 XP) · a companion, and not a combatant
AC 12 (oiled leather and a cork float) | HP 22 (4d8 + 4) | Speed 30 ft., swim 40 ft.
Skills Athletics +3, Perception +4, Survival +4 · Passive Perception 14 · he holds his breath for 6 minutes, works in black water by feel, and cannot read a word
Will Not Go Below the Flood Line. He stays above the water line and never comes within 30 ft of the crack. He cannot be ordered, argued, shoved, or shamed past that, and he will say why in one sentence. Anything that puts him there sends him back up the slope on his next turn, at a run.
Knows the Wheel. Halls like this all drain the same way, and he has turned this pattern of sluice before. He knocks the pawl off the ratchet first, so the wheel opens in two rounds of Athletics DC 14 rather than three, for whoever turns it. He can grind out one of those rounds himself at +3; a round he turns lowers the water and never counts as one of the party's successes.
Reads the Water. At the start of each round he says what the room does next: which way it drains, where the level will be, and — one round before it happens — that the wall is about to give. He is right every time. He has no idea what is on the far side of it and does not want to be told.
Six Minutes of Air. A sponge-diver's lungs. Left below a flood line he does not drown: he goes down the nearest bore and comes up somewhere else, alive, later, and finished with whoever left him. Something that grapples him underwater is a different matter, and he knows it.
Will Not Fight. He carries a bronze bailing scoop and swings it once for 1d4 if something is actually on him. He has no place in a melee, will not put a hand on any part of a monster, and cannot break a grapple, his own or anyone else's.
On the Door. He climbs to an exit, throws the bar, and holds it open. While he is on the door no other creature has to spend an action on it, and he holds it until the last of the party is through.
Get Him Up. He drags one willing or unconscious creature within 5 ft up to 15 ft toward higher ground, clear of water it cannot stand in. He cannot move a creature that something else is holding.
Creature Cards
The arm is unique to this chamber. The dead it washed in with are Deep Water Kin (W14) and recur across the deck, so their numbers live in the bestiary.
Shared across rooms, so the full cards live in the bestiary: Drowned Hoplite. Every mention above opens the numbers here on this page.
The Arm Through the Wall
Gargantuan monstrosity, unaligned · CR 11 (7,200 XP)
AC 16 (rubbery hide) | HP 189 (14d20 + 42) | Speed 0 ft.
Saves Str +12, Con +7 · Skills Athletics +12 · Resistant to cold and to nonmagical bludgeoning · Immune to charmed, frightened, prone, blinded · Blindsight 60 ft.
It Is Only An Arm. This card is one limb of something much larger on the far side of the wall. It cannot be killed, only severed or driven off. At 0 hit points the limb withdraws through the crack; the creature it belongs to is unharmed, uninterested, and not statted.
The Pinched Sinew. Where the crack squeezes the limb, a band of white sinew is exposed. It is AC 20, and 50 slashing damage dealt to it within a single round severs the arm outright whatever its hit points are. Cold damage in the same round halves the total needed. Anyone within 20 ft of the wall can spot it with a DC 14 Investigation check or passive Perception 15.
Feels the Floor. It knows the position of every creature touching the water or the stone within 40 ft. Invisibility and darkness do nothing. Standing on rubble, on a column, or on the dry stair hides you from it completely until you attack.
Legendary Bearing. It takes two legendary actions per round, spent at the end of another creature's turn, and regains them at the start of its own.
Multiattack. Two tentacle attacks, or one tentacle attack and one use of Drag Under.
Tentacle. Melee, +12 to hit, reach 30 ft (40 ft in Phase 2), one target. Hit: 24 (3d12 + 8) bludgeoning damage, and the target is grappled (escape DC 18) if it is Huge or smaller. The arm can hold one creature, or two in Phase 2.
Drag Under. It pulls each creature it is grappling 20 ft toward the crack. A creature dragged into water 5 ft deep or deeper begins holding its breath, and a creature pulled to the crack itself is pinned there and takes 14 (4d6) bludgeoning at the start of each of its turns until it escapes.
Pull Down a Column. It wraps one column within reach and breaks it. Every creature in a 10-ft line from the column makes a DC 15 Dexterity saving throw, taking 22 (4d10) bludgeoning damage on a failure or half as much on a success. The rubble becomes difficult terrain and dry footing.
Ink and Surge (recharge 5–6). Black water floods out of the crack in a 30-ft cone. Each creature in it makes a DC 17 Strength saving throw, taking 21 (6d6) bludgeoning damage and being pushed 15 ft and knocked prone on a failure, or half damage and no push on a success. The area is heavily obscured until the end of the arm's next turn.
Sweep the Fallen. When a creature within reach stands up from prone, the arm makes one tentacle attack against it.
Coil and Test. It moves the coil of its length up to 20 ft along the floor, and every creature whose space it passes through makes a DC 16 Dexterity save or is knocked prone.
Widen the Crack. Costs 2 actions. It braces and shoves. The crack tears another foot open, the water rises 1 extra foot immediately, and any pin driven into the wall this encounter pops out.