Erinyes

An erinyes on the party's side is a transactional alliance, never a friendship. Maybe a warlock PC has a fiend patron who lends her for one job. Maybe an archdevil owes a favor to a party patron and the erinyes is the payment. Maybe the party has produced a contract she is bound to honor. Whatever the setup, name the terms in writing before she lifts the sword. She will follow the contract exactly and nothing else.

In combat, run her cold. Entangling Rope on the most dangerous enemy in the first round, then three Withering Swords on whoever the contract names as the priority. She will not heal a downed PC and she will not pull a punch on a target the party wants alive unless the contract says so. If a player asks her to spare an enemy mid-fight, she looks at the contract, then at the player, then continues swinging.

Out of combat she speaks Infernal and uses telepathy 120 ft., which makes her a useful interpreter in lower-planar negotiations. She will not lie about the terms of her contract, because a contract violation sends her home in disgrace. She will absolutely lie about everything else. Treat any conversational answer she gives as suspect unless it is technically true under the letter of the deal.

The clock is the price of having her. The moment the contracted task is finished, she leaves. If the party tries to extend the arrangement, she names a new price, usually something a paladin would refuse and a warlock would consider.

An erinyes is a contract enforcer in armor. She is sent, not summoned, and she does not arrive until the target has been formally named. Open the encounter with that: a scroll unrolling, a name spoken in Infernal across a dead courtroom, a sigil burning into the floor before she descends through it. The party should know who she is here for before she swings.

Lead with Entangling Rope. DC 16 Strength save, 4d6 force, Restrained until the rope is destroyed (AC 20, 90 HP) or the erinyes uses a Bonus Action to release. Pick the party member most likely to ruin the fight: the cleric who casts Banishment, the bard with Counterspell, the monk who closes ground. Once they are tied up, fly to her primary target and open with Multiattack: three Withering Swords at +8, averaging 24 each on a hit. With 178 HP, AC 18, and Parry adding +4 against one melee hit per round, she can out-trade most martial PCs.

Use the air. Fly is 60 ft., so retreat upward whenever a wizard prepares a spell with a 30-foot reach. Truesight 120 ft. ignores invisibility and Magic Resistance gives Advantage on saves against spells, so the party's control toolkit will mostly fail. Telegraph this with the first failed Hold Person, then let the players adjust.

Diabolical Restoration means an erinyes killed on the Material Plane respawns in Hell with full HP. She does not panic and she does not bargain. If the party drops her, she will return weeks later with a longer name on the contract and probably a backup. Make sure the table understands this before they celebrate.

Have the contract physically present. A folded parchment in her belt, a brand on her shield, a binding tattoo across her forearm. When a player asks "why is she doing this", the answer should be a thing they can read, take, or burn.