Roper

A roper that fights for the party is barely conceivable, so commit hard to the setup. A duergar slaver brought the roper as a guard creature and the party freed it. A drow priestess used a roper as a temple sentinel and a True Polymorph mishap left it bound to the party's warlock. An aboleth pact gave the players a thrall they didn't ask for. The roper does not like the party. It has been compelled, mostly through hunger management, and its loyalty extends as far as the next meal.

In play, the roper is a single-room ambush weapon. Set it in a corridor or chamber the party expects enemies to enter, brief whoever else needs to know not to step into the long tentacles, and wait. When the patrol arrives, the roper grapples two of them with tentacles at +7 reach 60 ft., reels them in for two bites at +7 for 17 piercing, and the rest of the patrol panics. Even one round of that is a fight-ender for an unprepared squad. Out of the chosen room, the roper is useless: 10 ft. walk speed means it cannot keep pace with a marching party, and its profile gives the party away the moment any town watch sees it.

Feeding is the loyalty contract. The roper expects living prey on a regular schedule. The party can provide it, lie about providing it, or put the roper down before it provides for itself. That choice is the actual ally arc.

Have the roper make a low cave-rumble noise the night before it expects to be fed. Players will start tracking the schedule on their character sheets and flinch when the noise comes early.

A roper is the cave that's already eating you. CR 5, 93 HP, AC 20, 10 ft. walk and 20 ft. climb, Spider Climb, Stealth +5, passive 16, darkvision 60 ft. It looks like a stalagmite until it doesn't, and the moment it acts the party realizes the room has been reaching for them since they walked in. Pick the chamber first: a forest of pillars, a ceiling full of spikes, anywhere the roper can hide in plain sight against terrain that already looks like it.

Open with two Tentacle attacks at +7 reach 60 ft. On hit, no damage, but the target is Grappled (escape DC 14) and Poisoned for the duration of the grapple. Each tentacle is its own AC 20, 10 HP target with Poison and Psychic immunity, and a destroyed one regrows next turn for free. Then Reel pulls every grappled creature 30 ft. straight toward the roper. Then two Bite attacks at +7 for 17 piercing each on whoever Reel dragged into reach. That whole sequence is one Multiattack. Read it again. The encounter is a vacuum that ends in a meat grinder.

Target choice is everything. A grappled caster who tries to cast loses Concentration the moment they take damage and has Poisoned disadvantage on attacks if they try to cantrip free. Yank the casters first; the fighter can hack at tentacles, but each is AC 20 and breaking one accomplishes nothing because it regrows. Roper does not flee. It eats until something kills it. Reward parties that retreat and come back with fire or any area effect that hits the body without needing to clear the tentacles first.

Don't roll initiative the moment the party enters; let them spend 30 seconds describing the cavern, then have a tentacle slap up from a stalagmite the cleric just leaned on. The first round should feel like the room itself attacked.