Vrock

An allied vrock is a stretch and you should set it up clearly. The framing that holds: a more powerful Abyssal patron has loaned the vrock to the party for one specific job, usually the murder of a rival demon or the destruction of a celestial outpost the patron wants gone. The vrock arrives, executes the job, and leaves. It does not speak Common, only Abyssal and telepathy 120 ft., and it telepathically complains about every PC's smell for the entire mission.

In a fight on the party's side, run the vrock as a fragile heavy. Stunning Screech is now an alpha strike on the enemy formation: 1/Day, DC 15 Con, 20-foot emanation, Stunned for a round. Demons in the emanation succeed automatically, so use this away from any allied fiends. After the screech, the vrock flies in for Multiattack: two Shred at +6 for 20 each on the highest-AC enemy. Spores (recharge 6, DC 15 Con) lands on the densest enemy cluster and locks them out of melee with the rest of the party for the next minute, which is enormous in a multi-stage fight.

The vrock will not protect anyone but itself. If the cleric goes down next to it, the vrock keeps attacking the enemy in front of it. If the party orders it to retreat, it ignores them; demons do not take orders from mortals, only from the patron who wrote the contract. Once the contract job is done, the vrock leaves immediately and the party should hear it laughing telepathically as it flies away.

Have the vrock telepathically tell one PC, before it leaves, that it remembers their face. Demonic Restoration means it is alive forever. The party will not forget the threat.

A vrock is a vulture-shaped area-denial demon with a screech that breaks formations and spores that punish anyone who closes. CR 6, 152 HP, AC 15, fly 60. Pair them up against tier 3; one vrock alone bullies a level 5 party but evaporates against level 8 unless you give it terrain.

Open with Stunning Screech if the party is bunched. 20-foot emanation, DC 15 Con, 10 thunder on a fail and Stunned until the end of the vrock's next turn. Stunned removes actions and reactions and auto-fails Str and Dex saves, which pulls targets out of the next round. This is 1/Day, so use it round one on the cleric and a frontliner in the same emanation. After that, fly up 30 ft. and drop Spores (recharge 6) into the densest cluster: DC 15 Con, Poisoned and 1d10 poison per turn until they save out, ended early only by emptying a holy water flask on them. Most parties don't carry that. They will after this fight.

Multiattack is two Shred at +6 for 20 each (10 piercing and 10 poison). Use the fly speed to dive on whoever the party least wants to lose, then pull back up to 60 ft. by end of turn. Magic Resistance gives Advantage against Hold Monster and Banishment; make those rolls openly. At Bloodied, the vrock does not flee. Demonic Restoration means death is a setback, and it fights until it drops, screeching insults in Abyssal the whole way.

Tell the party, the first time Spores hit, that one PC's skin is now growing greenish patches that pulse. Don't roll the d10 silently. Roll it in front of them at the start of each of that PC's turns until somebody finds a holy water flask.