Lich

A lich ally is the rarest framing in the bestiary, and it should never feel safe. The setups that work: a lich older than the current cosmic threat is willing to cooperate against something it considers a threat to its own existence. A lich's phylactery is in the party's possession and the truce is enforced. A lich is the party warlock's patron and has decided to participate personally for one scene. In all three cases the party should know that the alliance ends the moment the lich's interests change, which is any moment.

Negotiate the terms in scene. The lich speaks all languages and has Insight +9, so it knows when the party is lying. Have it ask for one specific thing in return for its help: a soul, a relic, a name, the location of a rival's phylactery. If the party refuses, the deal is off. If they accept, write the price down and make sure the player understands they'll be paying it later.

In combat the lich is overwhelming on the party's side. Open with Fireball or Lightning Bolt (both level 5, both at-will) on whatever the party has identified as the priority target. Save Finger of Death for a creature the lich wants raised as a thrall (you can have the lich claim the corpse afterward, which is the kind of unsettling that should make the cleric uncomfortable). Animate Dead is the lich's bonus contribution to the fight: ten zombies from the bodies of the previous encounter, all under the lich's control. Counterspell the enemy caster's first big spell, Shield the lich itself when a stray attack comes in.

The lich will not heal the party or coordinate with them. It does its work and watches them do theirs, and when the fight is over, it leaves with whatever it came for. If a PC died during the fight and the lich finds the body interesting, it takes the body. The party can object, but the lich has Power Word Kill and they have a long road back to town.

When the alliance ends, the lich becomes the next campaign's villain or the current campaign's secret co-conspirator. Either way, the players should never relax around it. Write down what the lich saw the party do, and have it use that information against them later.

A lich is a campaign-ending threat that should never be a single combat. CR 21, AC 20, 315 HP, Initiative +17, and a Spirit Jar that resurrects the lich in 1d10 days if killed. The fight is the punctuation; the campaign is the sentence. By the time the party reaches the lich, they should know what its phylactery is, where the lair is, and what bargains it has tried to make. Otherwise the party kills it and it comes back, and the players feel cheated.

Open from initiative 27. The lich almost certainly acts first and uses Spellcasting to cast Fireball (level 5) at the densest cluster, DC 20 Dex, or Lightning Bolt (level 5) along a line if the party stretched out coming through the door. Save Power Word Kill for a PC the party will want to revive: someone under 100 HP who's about to drop from soak damage. Finger of Death is your second-round opener if the party survived the first one. Multiattack with three Eldritch Bursts at +12 for 31 force each (range 120 ft.) means the lich never needs to be in melee, but Paralyzing Touch is the trap card: if a fighter closes, the lich switches to one Paralyzing Touch (15 cold plus Paralyzed until start of next turn) and watches the rogue crit-fish on the prone caster.

Legendary actions are the encounter's tempo. Disrupt Life on a turn where three or more PCs are within 20 ft. of the lich is 31 necrotic to each on a fail (DC 20 Con). Deathly Teleport breaks grapples and repositions outside the cleric's aura, with 2d10 necrotic to anyone the lich was standing next to. Frightening Gaze (Fear) is your answer to the paladin who keeps closing. Protective Magic is the reaction that wins the fight: Counterspell the cleric's first big upcast, Shield the rogue's Sneak Attack crit. Save the four Legendary Resistances for save-or-suck spells (Banishment, Hold Monster, Wall of Force boxing the lich in).

Liches do not flee, they relocate. Plane Shift (2/day) and Dimension Door (2/day) let the lich step out of a losing fight and hit the party three sessions later from a different angle. Truesight 120 ft. and Passive Perception 19 mean Invisibility, Disguise Self, and most illusions don't work on the lich. Tell your players this before the fight. Mid-combat, "my Greater Invisibility is doing nothing" reads as a GM gotcha; in scene one of the arc, it's a problem to solve.

Have the lich call one PC by their childhood name. Not their adventuring name, the one their mother used. The lich researched them. The party should feel observed.