Gelatinous Cube

A gelatinous cube as ally is a stretch, but a real one. The framings that work: a wizard's pet research subject that the party has been asked to escort through a sewer, an alchemist's mobile waste-disposal unit hired for the night, or a cube that an oozemancer druid has bound with a season-long ritual. In all three cases, the cube is not a friend. It is a slow, hungry tool, and your job is to keep it pointed at the right corridor.

Run the alliance as logistics. The cube moves 15 ft. per turn, has a -4 initiative modifier, and will eat anything organic that fails its DC 12 Dex save, including the party's prisoner, the party's familiar, and the rope the rogue is climbing. Position is everything. Have the party negotiate marching order out loud. The cube goes last, in a corridor wide enough that nobody backs into it, and the wizard who is "controlling" it makes an Arcana or Animal Handling check (DM's choice) every couple of rounds to keep it on task.

In a fight, point the cube at the densest enemy formation and let Engulf do its work. Three failed DC 12 saves and the front line is Restrained, suffocating, and out of Counterspell range. Pseudopod cleans up stragglers at +4 for 12 acid. The Transparent trait means enemies who have never seen the cube give it a free first round; describe the goblin charging straight into nothing and disappearing waist-deep.

When the job is done, the controller dismisses, leashes, or relocates the cube. Anyone who tries to hand-feed it loses the hand.

Roll the DC 12 escape checks for any swallowed prisoner in the open. The party should hear the dice clatter and decide whether to spend a turn pulling them out.

A gelatinous cube is a hallway with teeth. It exists to punish parties that march in single file through dungeons they have not scouted, and the encounter is mostly a Perception puzzle wrapped around one nasty grapple. The Transparent trait sets a DC 15 Wisdom (Perception) check for any character who has not yet seen the cube move. If the marching order leads with a low-Wisdom barbarian and a passive 11, that barbarian walks into 10 ft. of acidic jelly and rolls Engulf at Disadvantage on the way in.

When initiative starts, the cube acts on its dismal +6 score and uses Engulf rather than Pseudopod whenever a target is in its path. Engulf moves it up to 15 ft. without provoking opportunity attacks, forces a DC 12 Dex save on every creature whose space it enters, and on a fail deals 10 acid plus the Restrained, suffocating, no-verbal-spells package. A Medium PC who fails is now taking 10 acid at the start of every cube turn and rolling DC 12 Athletics to escape, while losing access to Bless, Healing Word, and Counterspell. If the cube has nowhere to go, fall back on Pseudopod for 12 acid at +4.

Vulnerabilities are the puzzle. The cube has 63 HP, AC 6, and immunities to Blinded, Charmed, Deafened, Exhaustion, Frightened, and Prone. None of the standard control spells land. Damage is the answer, and the answer is loud: a level-2 Scorching Ray solves the encounter in one round. The horror is for the engulfed PC, not the party. An ally trying to pull a friend out makes a DC 12 Athletics check and eats 3d6 acid for the rescue, which is a real choice for a wounded cleric.

Cubes do not retreat. They have a 1 Intelligence and they are already at lunch. The fight ends when the cube ends.

Place a corroded breastplate, a half-eaten boot, and a clean-picked skull just inside the corridor. The party that reads the room sees the cube. The party that does not, becomes the next set of contents.