Lemure

A lemure as ally is a hard sell, but it works as a warlock's lowest favor from an infernal patron, a witch's bound thrall held in a containment circle, or a salvage object the party has hauled out of Hell for some scholar who pays per pound. The lemure is not a friend. It is barely a tool. Frame it as a rented appliance that leaks.

In combat, the lemure does almost nothing useful. 20 ft. speed, +2 to hit for 2 damage, AC 9. What it can do is soak up an attack of opportunity, walk into a Fireball, or stand in a 5-foot square the party needs occupied for a single round. Fire and poison immunity make it the perfect creature to send into a burning room or a poison gas trap to retrieve an object. It will not understand instructions beyond simple gestures (it understands Infernal but cannot speak), so the warlock or whoever owns it has to point.

The reason to keep one is interrogation theater. Threatening to discorporate a lemure in front of a higher devil makes the higher devil flinch, because every lemure is a soul mid-transformation and destroying one in a way that prevents Hellish Restoration insults the entire infernal hierarchy. Use the lemure as leverage in a parley, not as a fighter.

Have it leave a slime trail wherever it goes. The party will get tired of it before the GM does, which is the point.

A lemure is the lowest rung of devil and the encounter's atmosphere, not its threat. CR 0, AC 9, 9 HP, Vile Slime at +2 for 2 poison on a hit. One lemure is a speed bump. Eight lemures lurching out of a sulfur pit while a more interesting devil watches from a balcony is the encounter you actually want to run.

Run them as a wall of bodies, not a fight. The 20 ft. walk speed is glacial, so the lemures do not flank or maneuver; they shamble forward and press, and the party either holds the line or gets bogged down. Cluster them in groups of four to six so the cleric is tempted to spend a Turn Undead-equivalent (note that lemures are fiends, not undead, so the actual play here is something like Holy Word or a paladin's Smite). Resistance to cold and immunity to fire and poison cuts off the obvious area damage spells, so a wizard who reflexively casts Fireball gets a frustrating round.

Hellish Restoration is the GM's secret weapon outside combat. A lemure killed in the Nine Hells comes back in 1d10 days unless the killer was under Bless or the remains were sprinkled with Holy Water. Use this for set dressing in any infernal location: the lemures the party slaughtered last session are back next session, dragging themselves out of the same pit, and the party realizes nothing in Hell stays dead. That is more disturbing than any single combat.

The lemure does not retreat. It cannot. Int 1, Cha 3, no self-preservation. The fight ends when the lemures are all reduced to 0, and the party should feel like they are mowing down something pitiable. Describe them as faces half-formed, eyes weeping, mouths trying to plead in a language they no longer remember.