Magic comes from the warp, and the warp made mutated Gotrek who was a Dwarf, AND he had additional protection from magic. The Warp (and magic) can do anything, as long as the influence is powerful enough.
Also, I would honestly like to be critiqued for my char's lore problems. With WHFB dying and noone having memory of stuff like older editions, and thus slayer pirates it would be interesting to see what others have to say.