Unaware of the events that had transpired on Kiavahr and in the Raven Guard fortress-monastery of Ravendelve, Arendi was met with much distrust and scrutiny as some believed him to be a Traitor, but he was eventually cleared of all suspicion.
But Arendi kept a dark secret, until confronted by Corax himself -- he had callously used other Loyalist survivors of the Salamanders and Iron Hands as bait to seize a shuttle and escape the surface of Istvaan V alive.
Through rumours he had heard and suspicions he harboured, Arendi led the Raven Guard to the prison world of Carandiru, during a campaign which would become known as the "Day of Vengeance." Disobeying his initial orders, Arendi decided to divert his complement of warriors to rush to Corax's aid, who had been captured by the Traitors' new secret weapons, the genetically-altered Legionaries known as the "New Men," who were created by the Emperor's Children Chief Apothecary Fabius.
Arendi's timely arrival probably saved the Primarch's life, a favour the Ravenlord returned immediately when he spared Arendi the pain of being crushed to death by the last survivor of the "New Men."
Having fought side-by-side in battle and saved each other's lives, a new relationship of trust was established between the Primarch and his former bodyguard.
Arendi was later made the commander of the newly commissioned Black Guard, a Raven Guard unit which was formed from the Shadow Wardens, and commanded those forces during the Battle of Yarant.
He would go on to lead the newly-formed Black Guard Chapter as its first Master, following the sundering of the Legiones Astartes into separate Chapters during the Second Founding in 021.M31.