Cypher is the leader of the Fallen Angels. His true allegiance is unknown, as he is just as likely to help Chaos-aligned forces as he is to assist Imperials. He has constantly evaded capture by the Dark Angels for Millenia.
Official Canon Edit
TTS Canon Edit
Cypher decided it was time to do something "inexplicable" and lead the Adeptus Mechanicus to a planet in the Orior sector where giant ancient speakers were found. Satisfied that this would make the planet "rival Terra itself in its sacredness," he snuck aboard the Dark Angels' ship as they arrived to investigate Cypher's activities. Azrael himself mistook Cypher for another Dark Angel.