Board Thread:Warhammer 40k General Discussion/@comment-9676764-20140120182122/@comment-8881468-20140120230221

high amount of psyker mutation from gene-seed, Korvidae cult in the thousand sons (corvus= raven as familiy of birds)

for the rest, read the bloodraven's page, it is severly hinted at but not 100% made clear ever.