Demilich

In 3.5e, demiliches were extremely powerful, ascendent forms of liches. In 5e they're weaker and can't even cast spells. Obviously, this is unacceptable.

Demiliches are by definition epic-level spellcasters. Since we don't have proper epic-level spellcasting rules as of when I'm writing this guide, I'm using the spellcasting feature that Acererak has in ToA; I have no idea where WotC pulled the numbers from, but it's official, I guess.

If you're from the future and WotC has finally released official epic-level rules (or you have homebrew rules), the example demilich should cast spells as a 21st-level Wizard. I considered giving them completely unlimited spellcasting (like Quenthal Baenre from OotA), but that seems too powerful for just any old demilich.

Like with the variant lich in this guide, a demilich can be any spellcaster, not just a wizard. It can benefit from magic items that lay near its phylactery. It lacks any of the normal demilich special traits, but has many of those common to ordinary liches.