First: Since when was Ravannah's name Culwyeh? What are you basing this on? Culwyeh is/was a necromancer, we can assume, since he "fathered" the wights, but there is no evidence that Ravannah is a necromancer herself. Second: there is no reason to believe Ravannah had been killed prior to the Great War. All that is known is that Balor bound many Fallen Lords to his will and renamed them (who knows why: maybe ego, maybe to confuse the Light). In any case, we see this pattern many times, and the renaming has nothing to do with death. Connacht became Balor, Damas became Soulblighter, Myrdred became The Deceiver, Bahl'Al became The Watcher (that one is a bit speculative). My guess is that, considering that we know Balor, Soulblighter, and The Deceiver were all good in the previous age, Ravannah was originally also good, but turned to evil by Connacht/Balor before the Great War. She was killed by Rabican (nothing special there, just a weak Fallen Lord) and was resurrected by Soulblighter and forced to do his bidding. Perhaps in the process of dying and being resurrected, she lost some of her power. I don't see any holes in this, which is completely supported by the given evidence.
