Balor

A balor on the party's side is a deal with something that has Truesight 120 ft. and is reading every lie at the table. The framing that works is leverage: the party has something the balor wants (a bound rival's true name, a Sigil contract, a relic stolen from another Abyssal lord), and the balor is willing to spend an afternoon of its eternity solving the party's problem in exchange. Establish the price before any fight, and make sure the players hear it twice.

In combat, the balor is a one-creature siege engine and a hazard to its own allies. Fire Aura damages anyone in 5 feet, so the party fighter who runs in to flank is taking 13 fire per round from the balor's existence alone. Telegraph this. Have the balor say so in Abyssal through the bound translator, or telepathy through a willing party member, and let the party redesign the formation. Multiattack drops in front of the highest-priority target and disposes of it, then Teleport relocates 60 feet for the next problem. Flame Whip is a tractor beam for the wizard's chosen target.

The exit is non-negotiable. The instant the agreed task is done, the balor delivers the killing blow on whatever it was sent to kill, looks at the party once, and Plane Shifts back to the Abyss without ceremony. If the party tries to extend the contract, the balor names a new price that escalates by an order of magnitude. Slighting it, withholding payment, or laughing at it earns the party a personal entry in the balor's grudge ledger.

Have the balor refer to one party member by their childhood name. Truesight, but read as menace.

A balor is the encounter you build a campaign arc toward. CR 19 with 287 HP, Magic Resistance, and Truesight 120 ft. means the party should arrive understanding that they are about to do something very few mortals attempt. Sell the journey to the fight harder than the fight itself, because once initiative drops the balor does most of the work for you.

The opener is the room. Fire Aura ticks 3d8 fire at the end of every balor turn against anything in 5 feet, so melee characters take 13 free damage per round for being adjacent. Then Multiattack: Flame Whip at +14 with 30 ft. reach drags a Huge or smaller target up to 25 feet straight in and knocks them Prone (35 average damage), and Lightning Blade at +14 with 10 ft. reach denies the target Reactions until the balor's next turn (43 average damage). The combo means the rogue gets yanked into Fire Aura, knocked Prone for Advantage on the next attack, then hit by a sword that locks out Uncanny Dodge. Teleport as a Bonus Action jumps the balor 60 feet, which makes positioning kiting nearly impossible.

Save your Legendary Resistance (3/Day) for spells that actually decide the fight: Banishment, Plane Shift, Forcecage, Imprisonment. Magic Resistance gives Advantage on the rest. Cold and Lightning resistance plus Fire and Poison immunity means the elemental casters are working at half efficiency anyway.

Then the death twist. Death Throes is 9d6 fire plus 9d6 force in a 30-foot Emanation, DC 20 Dex save, 62 average damage on a fail. The party kills the balor and immediately rolls to survive killing the balor. Outside the Abyss, the balor reforms there with full HP. The win is real but conditional, which is exactly the note this fight should end on.