The Incontrovertible Truth once travelled to the Erioch System in order to deliver a consignment of Space Marines to Watch Fortress Erioch to begin tours of duty with the Deathwatch. It was then to travel to the planet Mariach and deploy a Kill-Team to deal with an Eldar raider incursion. However, as the Strike Cruiser dropped out of the Warp at the edge of the Erioch System, it was ambushed by ships of the Black Legion. Watch Captain Gharvil launched the Thunderhawk Gunships stationed on board and ordered the ship to fight back, but the Incontrovertible Truth was crippled and boarded by the Chaos Space Marines. Gharvil ordered the Marines on board to rendezvous at the port aft cargo hold to repel boarders, but many were either killed in the initial attack or scattered through the ship by the damage taken during the void battle.
Five Deathwatch Marines (Antor Delassio, along with Casella, Grennon, Lokar and Sanctimus) survived and set about trying to kill the Black Legionnaires as they made their way through the ship, setting up ambushes in critical corridors. Although they were able to kill many of the boarders, they began to suffer casualties. Grennon was the first to fall, followed by Lokar and Sanctimus with Casella badly wounded. Casella sacrificed himself to allow Delassio to escape and head for the bridge. As he made his way there, he realised that the Chaos Space Marines wanted the Deathwatch's ship intact so that they could bypass the defences of Watch Fortress Erioch and attack it. In a rage, Delassio stormed the bridge, killing all of the Traitors guarding it, and was able to throw the ship off course before succumbing to a Sus-anic coma from his wounds; he was the sole survivor of the attack.