Marilith

A marilith on the party's side is a cease-fire, not friendship, and the cease-fire holds exactly as long as her interests align with theirs. Demons do not bargain in good faith, but mariliths are tacticians, and a tactician will work alongside a useful enemy until the moment cooperation becomes inconvenient. Plausible setups: she has been bound by a high-tier diabolist the party rescued, she shares a target with the party (a balor, a lord of the Nine Hells, a celestial that humiliated her), or she is the unspoken patron behind a warlock at the table and the warlock is calling in a once-per-campaign favor.

Treat her as a hostile NPC with a contract. Telepathy 120 ft. and Truesight 120 ft. mean she knows when the party is lying and she will not pretend otherwise. State the deal in plain terms before initiative. She fights one battle, she takes one trophy, and she leaves. If the party tries to extend the arrangement, she Teleports out mid-conversation.

In combat she does what she does. Six Pact Blade attacks on the most armored enemy on the field, Constrict on the squishiest, Teleport to wherever the killing is densest. She will not coordinate with PCs and she will absolutely not protect them. Parry burns on hits aimed at her, never on hits aimed at allies. If a party member goes down inside her Constrict reach, she finishes the grappled enemy first. Demonic Restoration means she is happy to die for the kill if the kill is satisfying enough.

When the agreed task is done, she takes her trophy (a head, a sword, a name written in blood) and Teleports. Anyone who tries to stop her gets the next round of Multiattack.

Show the party the trophy clause in writing before the fight. They will hate it, and that is the correct emotion.

A marilith is a six-armed demon general who stands in the middle of a fight and lets the action economy do the work. The Reactive trait gives her a fresh Reaction every turn, Parry adds +5 to her AC against a melee hit, and her Multiattack is six Pact Blade strikes plus a Constrict in a single round. She does not need to move. The party brings the violence to her, and she returns it sixfold.

Open in melee. The Pact Blades hit at +10 for 17 each (10 slashing plus 7 necrotic), and six on a paladin is 102 damage in a clean round. Pile attacks on the hardest target until it drops, then spread the rest across the cleric and the rogue. After the blades, Constrict triggers DC 17 STR on the smallest creature in 5 ft. for 15 bludgeoning, grapples (escape DC 14), and restrains. A grappled wizard is a wizard you can stab six more times next turn.

Teleport is the encounter shape. As a Bonus Action on Recharge 5-6, she pops 120 feet to any space she can see. Use it the first time the party tries to gang up at one corner: vanish, reappear behind the cleric, six blades. Teleport again the moment it recharges, ideally right before a Hold Monster lands. Pair that with Magic Resistance and Truesight 120 ft., and casters cannot reliably control her.

Parry is the reaction every round. Save it for a Smite, a Sneak Attack, or a crit; +5 to AC turns most heavy hits into misses. Reactive means she still has her opportunity attack on top of that. Demonic Restoration is the GM's ace: if the fight goes badly, she dies, the body dissolves to ichor, and she reforms in the Abyss with full HP. The party "wins" and learns it solved nothing.

Have her name a player character on initiative one and address that PC, by name, between every round. Demons remember. The next campaign arc starts in the Abyss.