Ghoul

A ghoul ally is the necromancer-PC scene. The most plausible setup is a warlock or grave-domain cleric who has bound a single ghoul as a household guardian, or a party that has struck a temporary truce with a ghoul-led crypt because something worse is moving in. The ghoul speaks Common, which is the door into roleplay; it remembers being a person and resents the question.

In a fight on the party's side, the ghoul is a control piece, not damage. Send it at the enemy spellcaster or skirmisher and Bite-Claw to land the DC 10 Paralyzed condition. A paralyzed enemy means the rest of the party gets advantage and any melee crit within 5 ft., so the ghoul's job is to set up the fighter, not to finish anyone itself. The Bite for 5 piercing plus 3 necrotic doesn't matter; the Claw save does. With Multiattack the ghoul can Bite then Claw, or Claw twice, depending on whether the target is non-elf and worth paralyzing.

Out of combat, the ghoul is a problem. It eats corpses on sight, and the party's NPC contacts will refuse to share a roof with it. The cleric who hired it will eventually be asked by their order to put it down. Let that conversation happen at the table.

If the ghoul is destroyed, do not respawn it. Ghouls remember their lives just enough to make their second death matter.

Have the ghoul ask, once, if the party knows what its name was when it was alive. Then never bring it up again.

A ghoul is the lockdown mook of the low-tier undead lineup. 22 HP, AC 12, Multiattack of two Bites or a Bite plus a Claw, and the Claw carries a DC 10 Constitution save against Paralyzed until the end of the target's next turn. The damage is unimpressive (5 piercing plus 3 necrotic on the Bite, 4 slashing on the Claw), but a paralyzed PC is autocrit territory for the second ghoul standing next to them. Run ghouls in packs of three or four for a low-level party and the math gets ugly fast.

Open in a graveyard, crypt, or sewer at night. Ghouls have Darkvision 60 ft. and Poison-immune plus Charmed-, Exhaustion-, and Poisoned-immune, so the cleric's first instinct of inflict-fear-and-run gets nothing. The opening turn pattern is: one ghoul Bite-Claws the closest PC to land the paralysis, the second ghoul Bites the same target twice for the autocrit (advantage on attacks within 5 ft. of a paralyzed creature, and any hit within 5 ft. is a critical), and the third ghoul tries the same trick on whoever moves to help. The wizard's Sleep won't work on undead. The bard's Hideous Laughter won't either. Force the party to actually trade hit points.

The Claw save targets only non-undead and non-elf creatures, so an elf in the party is the obvious cleanup hitter. Have the ghouls focus on the half-orc fighter and ignore the elf druid until last; the metagame inversion of "ignore the front line" is unsettling and tactically correct.

Ghouls have Intelligence 7 and an animal-cunning level of pack instinct. Once half the pack drops, the survivors flee deeper into the warren toward whatever ghast or wight is keeping them. They do not parley. They do not negotiate. They eat.

Roll Claw saves in the open and call the DC out loud. A 10 looks easy until someone rolls a 7, and the table groans on cue.