Luther was not banished because he let a nuke on the dark angels flagship. he was banished because he found out about it but did not tell anyone intill the last secound and when he went to the hanger deck to disarm the nuke librarian zachariah was suspicious of the nose of the shuttle and was inspecting it at the same time Luther was going to disarm the nuke. they were able to disarm it and Luther confessed that he was going to allow the nuke to go off so he could become the leader of the Dark angels. when the campain to conquer sarosh started and Luther, Zachariah, and the Lion went to the cave to kill the daemons possing as the god of sarosh by using a psyker bomb. But when the Lion saw the daemons kill his men with ease, the librarians were able to kill them with their pskyer power and the psyker bomb he feared the powers of the librarians under his command (including zachariah who he hailed as his brother because zacheriah was the only other man to kill a calaban lion other then lion el'jonson himself) and banished them and Luther back to calaban. it was then that Luther started to truly become jealous of the Loin