Sounds like like several people are describing a “death knight” or “dark templar”. I’ve been thinking about this for a while and I’d love to see a heavy armor class that uses dark magic /necromancer type spells to augment physical combat. Include auras, shouts and possibly a minion or two.
With that said, the death knight/dark templar is pretty common in fantasy games but I have no idea what sort of lore GW has to support it.