Complex sentence: Tactical Sergeant Tarkus reveals that the traitor of the Chapter was Devastator Sergeant Avitus, and after his death Tarkus makes an oath of silence and is dubbed as "The Ancient".
Split into simpler sentences: Tactical Sergeant Tarkus reveals that the traitor of the Chapter was Devastator Sergeant Avitus. After he slew Avitus, Tarkus makes an oath of silence and is dubbed as "The Ancient".