The rock in a hard place
Mazo’s file has been purged by Inquisitorial Decree. It is also a known fact that he underwent a near complete mind wipe upon his entry into the Chassis of a Dreadnaught. But Mazo has been a faithful guard of the Deathwatch, and his many years of silent wisdom show by the age of his Dreadnaught model. In recent times, Brothers of the Deathwatch have looked to him as a symbol of hope and faith. Though Mazo is seen as an inspiration, the Master of the Watch has not let him see battle in centuries, perhaps out of fear of losing him.