Archmage Ghost (CR 5)

Rejuvenation. If it has a phylactery, a destroyed lich gains a new body in 1d4 days, regaining all its hit points and becoming active again. The new body appears within 5 feet of the phylactery.

Incorporeal Movement. The ghost can move through other creatures and objects as if they were difficult terrain. It takes 5 (1d10) force damage if it ends its turn inside an object.

Magic Resistance. The archmage has advantage on saving throws against spells and other magical effects.

Spellcasting. The archmage is an 11th-level spellcaster. Its spellcasting ability is Intelligence (spell save DC 15, +7 to hit with spell attacks). The archmage can cast disguise self and invisibility at will and has the following wizard spells prepared:

Cantrips (at will): fire bolt, light, mage hand, prestidigitation, shocking grasp

1st level (4 slots): mage armor*, magic missile, false life

2nd level (3 slots): detect thoughts, mirror image, misty step, blindness/deafness

3rd level (3 slots): counterspell, lightning bolt

4th level (3 slots): dimension door, fire shield, polymorph*

5th level (3 slots): cone of cold, wall of force

6th level (1 slot): create undead

*The archmage casts these spells on itself before combat.

Possession (Recharge 6). One humanoid or giant that the ghost can see within 5 feet of it must succeed on a DC 13 Charisma saving throw or be possessed by the ghost; the ghost then disappears, and the target is incapacitated and loses control of its body. The ghost now controls the body but doesn't deprive the target of awareness. The ghost can't be targeted by any attack, spell, or other effect, except ones that turn undead, and it retains its alignment, Intelligence, Wisdom, Charisma, and immunity to being charmed and frightened. It otherwise uses the possessed target's statistics, but doesn't gain access to the target's knowledge, class features, or proficiencies.
The possession lasts until the body drops to 0 hit points, the ghost ends it as a bonus action, or the ghost is turned or forced out by an effect like the dispel evil and good spell. When the possession ends, the ghost reappears in an unoccupied space within 5 feet of the body. The target is immune to this ghost's Possession for 24 hours after succeeding on the saving throw or after the possession ends.