Complex sentence: The military leader of the group, who used the name Randall Rader during his stay at CSA, left the group in a rift with Ellison and joined a newly forming group in Idaho called "The Order".
Split into simpler sentences: The military leader of the group, who used the name Randall Rader during his stay at CSA, left the group after a rift with Ellison. He joined a new group in Idaho called "The Order".