Hezrou

A hezrou ally is a deal with a devil, except worse, because the hezrou is dumber and angrier than a devil and has no concept of a contract that does not end in violence. Most plausible setup: a binding circle, a summoned creature held to a single task, or a relic that compels obedience for a count of rounds. Sell the leash. The hezrou should be visibly straining against whatever holds it, and the players should understand that the leash is going to slip.

In combat run the demon greedy. It does not coordinate. Three Rends per turn at +7 for 15 average each, plus Stench in a 10-foot aura that hits the party's melee characters as readily as the enemies. The DC 16 Constitution save is brutal at the levels where a hezrou might be available, so position carefully. The fighter is going to fail one of those saves and spend the next round at Disadvantage. Make peace with that before you summon.

The hezrou refuses subtlety. Scouting, parley, and mercy are off the menu, and any surrendering enemy gets eaten anyway. Its Int is 5 and its Charisma is 13, which makes it cunning enough to lie but not patient enough to follow through. If the party tries to use it for anything except a brawl, it gets bored and starts looking at them. Telepathy 120 ft. means it can mock them in their heads while it does so.

The exit is the binding ending. Once the timer runs out or the circle breaks, the hezrou turns on the summoner first if it can reach them. If it cannot, it leaves through whatever door is loudest. Demonic Restoration means it never truly leaves the table. The party has just made an enemy that cannot die.

A hezrou is a wall of stinking meat with 157 HP and a 10-foot poison aura that punishes anyone who stands next to it. AC 18 plus Magic Resistance plus 5 in every physical save means the standard control package (hold monster, banishment, polymorph) lands maybe one time in three. The party's answer is fast focused damage from outside the aura.

Stench is the entire tactical identity. Every creature that starts its turn within 10 feet of the hezrou makes a DC 16 Constitution save or has the Poisoned condition until the start of its next turn. Poisoned is Disadvantage on attacks and ability checks, so a fighter who fails the save is hitting AC 18 at Disadvantage. Position the hezrou where the party's melee characters cannot avoid the aura without giving up their multiattacks. The hezrou wants to be the door.

In open ground use Leap. The bonus action covers 30 feet for 10 feet of movement, so the hezrou's effective threat radius is the entire battlefield. Open with a Leap into the casters' line, follow with three Rend attacks at +7 for 15 average each (45 on full hits, with poison damage that bypasses most resistances), and force the party to spend a round either disengaging or eating the aura. Initiative +6 means it usually gets to choose first.

Hezrous don't retreat in the sense that mortal creatures do. Demonic Restoration means death just sends the body home to the Abyss with full HP, so the demon fights at full commitment until destroyed. The price for the party is that killing it once doesn't kill it. If the hezrou was summoned, the summoner has lost it for the scene. If it was wandering loose, the party should hear rumors of the same demon a month later, three planes away.

Describe the smell before the demon. The party should make a Perception check, fail it, and only then notice that the cleric has gone green.