Glabrezu

A glabrezu as ally is a Faustian bargain that is currently in its honeymoon phase. The most plausible setup is a warlock PC's pact patron showing up in person, or a party that has been forced into a temporary alliance with a demon to stop something worse, like a balor or a mind flayer cabal. The glabrezu helps because it is owed something specific, and the bill comes due later.

Set the price in the open. The demon agrees to one thing: kill this archmage, retrieve this artifact, hold this fortress for one night. In return, it wants something the party is going to regret giving up. The favor is the entire scene. Use telepathy 120 ft. to keep the negotiation private between the demon and one PC, and let the other players watch.

In combat with the glabrezu on side, run it gleefully cruel. The two Pincer attacks at +9 grapple anyone Medium or smaller, and the demon will absolutely grapple a downed enemy who could still be questioned and Pummel them into a smear. Power Word Stun goes on whichever enemy commander the party wanted to interrogate. Confusion drops on a tight cluster regardless of whether the party's barbarian is in the radius. The glabrezu does not coordinate or warn, and it never apologizes. Magic Resistance plus Truesight 120 ft. means it ignores most enemy magic and sees through every illusion.

When the favor is done, the glabrezu leaves. The price is owed. If the party tries to renege, Demonic Restoration means killing it accomplishes nothing. It will be back, and it will tell every other demon in the Abyss who cheated it.

Have the demon address the warlock by their pact name, the one nobody else at the table knows. Watch the player's face.

A glabrezu is a corruption demon that wins encounters by talking first. CR 9, 189 HP, AC 17, Truesight 120 ft., telepathy 120 ft., and Magic Resistance gives it advantage on every spell save the party throws. The pincers do the killing, but the scene starts with an offer. Have the glabrezu speak telepathically into the most ambitious PC's head before initiative and propose something specific: a magic item, a true name, a piece of forbidden knowledge. If the player bites, the fight may not happen at all, and the campaign now has a corrupted PC.

Once swords come out, the Multiattack is two Pincer attacks at +9 reach 10 ft. for 16 slashing each, and one of those hits grapples a Medium or smaller creature on either pincer (escape DC 15). After the pincers, the glabrezu uses Pummel or Spellcasting. Pummel is a DC 17 Dex save against a grappled creature for 15 bludgeoning, half on save. The play pattern is Pincer-grab on the rogue, Pummel them in place, then Pincer-grab the cleric next round. The grapple is the engine.

Spellcasting is your panic button. Power Word Stun once per day takes a player out of the fight if they're under 150 HP, no save. Confusion (DC 16 Wis) shuts down a clustered party. Darkness covers a retreat. Save Dispel Magic for the moment a wizard drops Hold Monster or Banishment. Fly is the escape route once it's bloodied.

Demonic Restoration means killing a glabrezu outside the Abyss is temporary. Tell the party. They should know the demon they just dropped is reforming somewhere in the Abyss right now, and the deal it offered the rogue is still on the table.

Speak the offer aloud in second person to the player, by name, while the rest of the table listens. The other players cannot intervene. That's the encounter.