Appropriate CR for a level 8 party

I'm writing an adventure for a party of level 8 PCs. I haven't ran anything this high level yet, and I wanted some advice about what would be an appropriate challenge.

The party consists of:

A human paladin/sorcerer A warforged warlock/bard A Kalishtar fighter/barbarian A changling bard/sorcerer And a shifter rogue/fighter

The story has them temporarily arriving in a domain of Dread in ravenloft, which is ruled by someone from one of the players backstory. I want to make the antagonist for the adventure an undead, but am unsure what CR would be an appropriate challenge or if there is a monster I can use instead with some reflavoring