Well, it would make sense from in-universe perspective no? You can already use Shyish to manipulate life-force or summon ghosts of dead people, Mortuary Cult was trying to invent true immortality for quite some time, there was plenty of Dhar using magicians before Nagash, from Beastmen, Troll hags to random Human tribesmen and Druchii who's teachings Nagash used for his magical tradition, would it really be a stretch for someone before Nagash to use Dark Magic with heavy emphasis on Shyish?
Also, Drachenfels kinda-sorta fits as a example, considering the fact he is still referenced in modern publications, but for this discussion that's just circular logic. Still for the sake of completeness I think I should point this out that you can technically count him if you are very charitable.