Priests of Tomb Kings or rather Nehekhara priests. They invented "immortality". The reason it didn't work for the Tomb Kings themselves were that they weren't priests themselves. (If I remember correct)
Cynical Cat, you're going in a loop. The texts you refer to are "persons" in-game. This has been said quite many times. The very reason GW uses these in-game persons to explain a lot of things is, that if they need to change something to suit there next version or new system they can say that "Oh, that was just a misguided view of one person." They do that. A lot. It's their game. It's their fantasy world.
Also, we are talking about magic that derives from Chaos. I find the current form of magic much too lawful for my taste. I see no reason why a priest couldnt be as powerful or even more powerful than a wizard. It's just another form of Chaos!
(Ah, I think I hear the Witch Hunters on my door, brb...)
).