What was the storyline behind Khull-Khumm (how he became evil) and how did Wahooka lose his title as the Shadow King?

I believe that some of this was answered in the book that came with stonekeep.

The book was called " Thera's Awakening " It was a sort of prelude to stonekeep.
There is also an epilogue sort of book for stonekeep called " The Oath Of Stonekeep "

If your are into stonekeep, I'd recommend getting it.