Nalfeshnee

An allied nalfeshnee is a creative stretch and you should name it. The most workable framing is a contract: a higher fiend, an archdevil's broker, or a Demon Lord with a grudge has loaned this nalfeshnee to the party for one specific Abyssal task, and the loan is over the moment the task is done. The nalfeshnee will not pretend to be friendly. It will execute the contract precisely and complain in telepathy about every PC's moral failings while doing it.

In a fight on the party's side, run it as a finisher. Teleport 120 ft. to wherever the cleric is being mobbed, drop Horror Nimbus (DC 15 Wis, 28 psychic, Frightened), then Multiattack the nearest enemy spellcaster for three Rends at +10. With Truesight 120 ft. it ignores invisible enemies, so point it at whatever the party can't see and watch the problem evaporate. Pursuit will reflexively chase any enemy who tries to teleport away. Tell the party in advance that the nalfeshnee will not heal, will not protect, and will not save anyone but itself.

The exit is the danger. When the contract ends the nalfeshnee leaves immediately, and anyone who insulted it during the work is logged for later. Have the demon name a single PC at the end and tell them, in their head, that it will see them again. Then it teleports out. That promise is the whole reason this team-up was worth running.

A nalfeshnee is a 184-HP teleporting boar-headed inquisitor that wants the party to feel small before it kills them. Truesight 120 ft. cuts through every illusion, hide, and Invisibility on the table. Telepathy 120 ft. means it talks to whichever PC it has decided is the worst person in the room. The fight begins with a sentence in their head and a 120-foot Teleport into the middle of the formation.

Open with Teleport into reach of two or three PCs at once, then Multiattack: three Rend at +10, reach 10 ft., for 27 average each (16 slashing and 11 force). That's a potential 81 damage in one turn, enough to drop most level 9 frontliners. If Horror Nimbus is up (recharge 5 to 6), use it as a bonus action right after the Multiattack: 15-foot emanation, DC 15 Wis save, 28 psychic on a fail and Frightened for a minute or until line of sight breaks. Force and psychic together hit through most resistances built around fire and necrotic, which is the whole point of this monster.

Pursuit is the lever that breaks escape attempts. Whenever a creature ends its move within 120 ft., the nalfeshnee can Teleport to within 10 ft. of it as a reaction. The wizard who Misty Stepped away gets the demon next to them again, with all three Rends ready next turn. Use this once on the first runner, openly, so the party understands retreat doesn't work. Magic Resistance gives Advantage against Banishment, the one spell that ends the fight. Demonic Restoration means a kill sends it back to the Abyss with full HP, which is a campaign hook, not a defeat.

Have the nalfeshnee laugh into one PC's mind every time another PC fails a Wis save against Horror Nimbus. The Frightened condition is mechanical; the laugh is what they remember.