Ghast

A friendly ghast is a heavy stretch and you should commit to the framing in the opening beat. The plausible version is a necromancer NPC who has bound a ghast to a contract, or a paladin who has agreed to escort one home in exchange for information about a higher undead. The ghast remembers being someone. It speaks Common. It hates what it has become and will help the party for one specific outcome, then expects them to put it down.

In combat the ghast is a flanker the party has to position around. Stench Poisons everyone within 5 feet on a failed DC 10 Con save, including PCs, so the cleric and rogue cannot stand next to it without consequences. Send the ghast at the enemy line, let it Claw for paralysis on whoever the rogue calls, and keep the rest of the party at 10 feet of standoff. Necrotic resistance and Poison immunity mean it can absorb hits casters cannot, especially against other undead.

Out of combat the ghast is unwelcome everywhere. Towns close their gates. Temples will not open. Anyone who scryed on the party is now scared of the party. Make the social cost real. Every NPC interaction has the ghast lurking at the edge of the firelight while someone explains why the humans brought one of those into a market.

Promise the ghast its rest at a specific moment, then keep the promise. The party should remember the look on its face when the cleric finally swings.

A ghast is a ghoul that learned to hunt in groups. The defining feature is Stench, which Poisons anyone who starts their turn within 5 feet on a failed DC 10 Con save. That means the ghast wants to be in melee, plural, with as many PCs adjacent as possible. It is a melee enabler whose job is to lock down the front line so the ghouls or skeletons in the same room can chew through the back.

Open by closing. The ghast has 30 ft. walk and AC 13, which is not enough to soak focus fire, so it should pop out of cover already adjacent to two PCs. Use the Claw action over the Bite when you can, because Claw forces a DC 10 Con save against Paralysis on top of 10 slashing. A paralyzed PC is automatically Failing strength and dex saves and any attack inside 5 feet is an auto-crit. Bite is the cleanup option for 7 piercing plus 9 necrotic when you do not need the rider.

Stack the room. Two ghasts plus a handful of ghouls, or one ghast leading skeletons with bows from cover, gives you a fight where the players spend rounds dealing with paralysis and Poisoned disadvantage on attacks while the missile fire keeps coming. Necrotic resistance and Poison immunity mean a lot of low-level wizard tricks (Ray of Sickness, Poison Spray) just bounce. Tell the players the smell hits before initiative so anyone with a held breath option can use it.

Ghasts do not retreat. They were people once and the hunger does not allow restraint. Once Bloodied (18 HP) the ghast switches its target to whoever has done the most damage to it personally and ignores everything else. Play it as a fixation, not a tactic. The PC who was about to disengage gets one more Claw to the face.

When the party walks into the lair, describe the smell, then ask the highest-passive PC to make the save before any of them roll initiative. The fight starts already inside the cloud.