Chuul

A chuul ally is a hard sell, but the framings that work are short and weird. A mind flayer or aboleth has loaned the party a chuul as a one-time bodyguard for a swim into ruins they jointly want robbed. A young wizard inherited the chuul along with a tower and an instruction sheet that says "feed it the trespassers." A druid in a coastal grove has tamed one and uses it as a heavy lifter. The chuul never likes the party. It tolerates them because someone with more authority told it to.

Mechanically the chuul is useful in exactly one scene type: an underwater approach to an enemy position. Swim 30 ft. is slower than the party's likely Water Breathing pace, so let it scout ahead with Sense Magic and signal back when it finds something magical. In combat it Pincers and uses Paralyzing Tentacles to neutralize a single enemy spellcaster, which is a clean assist. Two Pincer grapples plus the paralysis lock down one enemy at the cost of the chuul's whole turn.

Out of combat the chuul is a roleplay liability. It only understands Deep Speech and can't speak. It will eat unattended food, pets, and small NPCs. It does not sleep on cue. The party should always feel one bad order away from a casualty.

When the loan ends, the chuul leaves and the party sees it once more in the campaign as someone else's enforcer. Make sure they remember it.

A chuul is an ambush predator with a specific job: grab a magic-using PC, paralyze them, and drag them somewhere wet. It will not negotiate, and it does not care about the rest of the party except as obstacles between it and the next item that pinged its Sense Magic.

Set up the encounter in or near water. Open with the chuul submerged and using Sense Magic (120 ft.) to pick a target before initiative starts, then reveal it as the wizard steps onto a dock or wades through a flooded chamber. Multiattack is two Pincer attacks (+6, 9 bludgeoning) and Paralyzing Tentacles. Lead with both Pincers on the chosen mark to lock down a grapple (escape DC 14), then trigger Paralyzing Tentacles. DC 13 Con save, and on a failure the target is Poisoned and Paralyzed, repeating saves at end of turn for up to a minute. A paralyzed PC eats crits at 5 ft.

Once a target is locked, the chuul moves toward water with the grappled creature in tow. The other Pincer is still free, so a second PC who closes to rescue gets grappled too, but the chuul will not waste Paralyzing Tentacles on them while the original target is still going. Poison immunity and Poisoned condition immunity make the obvious counter (a quick Poison Spray) useless.

Chuuls don't rout. INT 5, no morale worth running. The fight ends when the chuul is dead or the chuul is gone with someone. If you don't want a TPK risk on the rescue, give the players a clear escape path: a flooded passage with an air pocket, a noise that draws the chuul off the captive after a round.

Telegraph Sense Magic the round before the ambush. Have the chuul's eye stalks twitch toward the cleric's holy symbol just before it lunges. The players should know which feature is hunting them.