Darkmantle

A darkmantle does not ally. It has Intelligence 2 and the social range of a barnacle, so the framing has to be a handler. The cleanest version is a drow scout, a deep gnome trapper, or an Underdark guide who has trained one to roost on a shoulder and drop on command. The party hires the handler. The darkmantle is the handler's tool, the way a hawk is a falconer's tool, and it answers to one whistle.

Use it for one trick: kill the lights in a room before the party kicks the door. Darkness Aura is once per day, so the handler will charge for the use, and the price is the entire reason the scene has tension. Send the darkmantle in first, let it darken a 15-foot emanation around itself, and the party charges into a blackout that none of the enemies have prepared for. This is the cheap version of a third-level Darkness spell, and the enemy spellcaster will not be ready for it.

The Crush attack is a side benefit. If a target enemy is Medium or smaller and the darkmantle gets surprise, the handler will direct it at the back-line caster, which covers, Blinds, and suffocates that caster while the rest of the party deals with the front. The party should not expect coordinated tactics beyond the initial drop. Once the trick is done, the darkmantle returns to the handler's shoulder and the handler taps it twice and feeds it something twitching.

The handler is the scene. The darkmantle is the gimmick. If the party loses the handler, the darkmantle flies off into the nearest dark corner and is just another encounter waiting to happen.

Have the handler refuse to discuss the darkmantle's name. The party should suspect it does not have one, and that should bother them.

A darkmantle is a ceiling trap with stats. Its job is to land on a face, kill the lights, and make a fight that should have been trivial into a panic. The setup is everything: a darkmantle on initiative across an open room is a CR 1/2 nuisance, but a darkmantle that drops from a 30-foot ceiling onto the wizard with surprise is a real problem for one round, which is exactly how long it should take to be one.

Lead with Darkness Aura. It is once per day, so spend it. A 15-foot emanation of magical darkness that even Darkvision cannot penetrate, lasting up to ten minutes on the darkmantle's concentration, drops the entire melee line into a blackout. Most parties have no answer to magical darkness at low levels. Move the darkmantle into the cluster of PCs immediately so the cleric and the rogue cannot see each other. On the next turn, dive on the heaviest-armoured target with a Stealth-from-darkness Crush attack, which has Advantage from being unseen, which means a Medium target gets covered, Blinded, and suffocating.

Treat suffocating as the timer. A creature can hold its breath for 1 + Con modifier minutes, then drops to 0 HP at the end of its next turn. Most low-level PCs have two to four minutes, which at six-second rounds is plenty of time for the party to free their friend, but the panic at the table will not understand that. The Athletics escape DC is 13, which a Strength PC clears comfortably. Anyone else needs an ally to spend an action helping. Make the players say out loud who is helping whom.

Darkmantles do not retreat. They have Int 2, Blindsight 60 ft., and a single instinct, which is to attach and crush. If the party kills the attached one, run any second darkmantle the same way and let the dice decide which PC gets a face full of cave-thing this time. They are pack ambushers in lore but mechanically they fight to the last, which is fine because the encounter is short either way.

Describe the Darkness Aura as the cave going wrong, not as a spell. The players know about Darkness. They should not realise the squid on the ceiling cast it.