Kel'thuzad by Rexigon

Level 20 - Skeletal Mages
Your skeletal warriors are now skeletal mages, that do more damage and attack from range.

Level 20 - Frost Apocalypse

Necromancer Kelt'huzad:
Kel'thuzad when he was still alive and not a lich.

Blood Lord Kel'thuzad:
Based off the Blood Lords from Wrath of the Lich King.

Mephisto Kel'thuzad:
Doesn't need to be exactly Mephisto, as he could be his own hero, but he could be some kind of floating demonic creature that summons demons.