Dretch

A dretch on the party's side is a leash situation. Some warlock, summoner, or unfortunate cleric has bound it, and now the party is stuck escorting a 20-ft.-speed Abyssal coward that resents every second of the arrangement. Lean on the comedy and the menace in equal measure. The dretch will obey the binding, but it will sandbag, it will whine in Abyssal, and it will use its telepathy 60 ft. to mutter complaints to anyone who happens to speak the language.

The one mechanical reason to bring a dretch is Fetid Cloud. It's once per day, a 10-foot Emanation centered on the dretch, DC 11 Con save, and on a fail the target loses either action or bonus action and can't take reactions until the end of its next turn. Used at the right moment, that's a brutal soft control: drop it in the middle of a knot of cultists right before the wizard's Fireball, and half of them lose half their turn. The party member holding the leash needs to decide when to spend the daily charge, since the dretch will not volunteer it intelligently.

In a straight fight the dretch is bad. Rend at +3 for an average of 4 damage doesn't matter. AC 11 and 18 HP doesn't matter. What matters is its resistances to Cold, Fire, and Lightning, plus immunity to Poison and Poisoned, which lets it walk through environmental hazards the party would have to detour around. Send it across the burning bridge first.

Make the binding visible. A chalk circle on the dretch's forehead, an iron collar, a tongue of red ribbon tied to a wrist. When the binding starts to fray, the party will notice, and the moment the dretch realizes it's free is its own scene.

A dretch on its own is a speed bump. A dretch in a pack of six in front of something worse is a real encounter. They exist to clog the front line, soak attacks of opportunity, and burn one of the party's spell slots on Fetid Cloud control before the actual demons arrive.

Run them as a pile. Five or six dretches at AC 11 and 18 HP each will eat about two rounds of party damage, which is two rounds the glabrezu or hezrou behind them gets to set up. Open with Fetid Cloud from one or two of them, ideally placed so the 10-foot Emanation catches three or four PCs at once. DC 11 Con is low enough that fighters and barbarians shrug, but the wizard and the rogue both need to roll. A failed save means action OR bonus action that turn, plus no reactions, which kills counterspells and shield castings.

After the cloud, dretches slap with Rend and absorb hits. Their resistances to Cold, Fire, and Lightning matter, since the party caster who reaches for an area spell loses half its value. Force the wizard to choose between hitting the dretches or the actual threat. Don't run the dretches as cowards in a pack; they are loud and stupid and slow at 20 ft. of walking speed, but they swarm because their abyssal lord told them to.

If the party engages the dretches and ignores the bigger demon, that is correct play and you should reward it by letting them clear the line. If they ignore the dretches, the dretches grapple and slow them while the real threat moves up. Either way, the dretches die. Their job is to spend the party's resources before they go.