I made a pretty nasty undead bbeg in a 3.5 game by applying both the lich and vampire templates to a necromancer, then giving him levels of the Vampire Lord prestige class from Libris Mortis. He also had the Swarm Shifter template, with swarm of undead parts. His skull would hover in the middle of basically a whirlwind of bones. I let the skull make Vampire drain attacks too, but also it was vulnerable to PCs trying to target it directly. The nastiest thing about him though was how many other undead he could control.

I mean, technically he is just a lich... Who so happens to have a cult that worships him, which somehow gave his incorporeal spirit power during his short absence after the defeat at the hands of his right-hand lieutenant, Kas, and has absorbed the power of a lesser deity to use as his own while escaping from imprisonment by the Lady of Pain for a failed plot to usurp the cosmic order when he infiltrated Sigil, the City of Doors.

> while escaping from imprisonment by the Lady of Pain for a failed plot to usurp the cosmic order when he infiltrated Sigil, the City of Doors. Man, Planescape: Torment was such a good game. Remake when?

https://www.gog.com/en/game/planescape_torment_enhanced_edition

What's the template from?

It's Question from the show Justice League Unlimited, probably with Lex Luthor under the Vecna face, so probably the episode "Question Authority".

"shouldn't be too hard"?! A regular lich is CR 21. For comparison, an adult red dragon is CR 17 and the most powerful demon, the balor, is CR 19. There are almost no creatures more powerful than liches. Even ones that are not Vecna.