Liches

Liches are like flameskulls in that most adventurers have heard of them at some point, though you'll find considerably fewer people in taverns claiming to have fought one. Liches are rare — not unlike true vampires. Think of how rare mages are. Now remember that archmages make up less than 1% of casters. Now remember that liches make up less than 1% of archmages. A lich is an immensely powerful caster who gained eternal life by embracing a state of undeath, and like most intelligent undead things, they are pretty universally bad news. Incredibly rare, immensely powerful, and notoriously hard to kill, they're the sort of creatures that even Strahd von Zarovich would think twice before poking.
The only lich the party has met is the Amber Temple Lich, who against all logic seemed to be relatively pleasant, helpful, and still invested in combatting evil.