Board Thread:Warhammer 40k General Discussion/@comment-25073042-20140731063418/@comment-24944355-20140731074408

The Alpha Legion stole the basic gene codex from which all the astartes were formed from The Raven Guard during the heresy as seen in Deliverance Lost. With that untainted source they can create an almost inexhaustible supply of legionaries with the bonus that the base gene codex can create marines much faster from almost anyone with very little in the way of rejection.