The following text contains plot spoilers.
Corporal Antioch is part of a detachment from the 203rd Cadian assigned to defend Manufactorium Ajakis and the Titan within. Captain Titus first encounters him through a transmission after the Ultramarines squad dropped a Titan cannon on an Ork force attempting to force their way into the Manufactorium.
Antioch directs the squad to a service elevator where the Imperial Guardsmen would meet the Ultramarines. Once the squad has arrived, Antioch brings up the lift. However, before it reaches the floor the Ultramarines are on, the Orks attack, and Captain Titus directs Antioch to halt the lift.
During the last wave of the attack, the lift reaches the Ultramarines and the Guardsmen onboard aid them in repelling the Orks. After the Orks were defeated, the Ultramarines squad boards the lift.
On the way down, Antioch informs Titus that all he can tell him is that the Orks have not breached the gate his force was holding and that Inquisitor Drogan was still inside and had not been seen in several days.
Titus informs Antioch that Lieutenant Mira's unit was establishing a bastion at Hab Center Andris, and suggests that they contact her. He is not encountered by Captain Titus again, and his fate is unknown.