Quick (and probably quite rough) attempt at Acererak:
Spoiler: Acererak, the Devourer
7th level Vestige
Vestige of Unlife. Acererak grants you a semblance of lichood while he is bound. You have resistance to cold, lightning, and poison damage.
Additionally, the undead see you as a friend while bound to Acererak. Before making an attack against you, an undead creature must make a Wisdom saving throw. On a failed save, the creature loses the attack. If a creature's saving throw is successful, the creature is immune to this feature for the next 24 hours.
Undead Healing. Whenever you take necrotic damage from a spell of 1st level or higher, you instead regain a number of hit points equal to the necrotic damage dealt, up to your hit point maximum.
You can cast the spell false life as if using a spell slot of 4th level. After casting this spell, you must take a short or long rest before casting it again.
Finger of Death. You can cast the spell finger of death without expending a spell slot
I'll add the flavor text in once I'm convinced I have the mechanics right. Finger of Death might be raising the level that this vestige needs to be at, but I'm unsure of where it needs to be otherwise. Lemme know what you think.
EDIT: Added a couple new features.