He took part in the Battle of Signus Prime, where the Archein's Company acted as the Legion's rearguard and was fully trusted by his Primarch to defended their flagship, the Red Tear, which had crashed onto the world. Disaster would strike in the battle, though, when Sanguinius was left unconscious after being severely wounded at the hands of the Bloodthirster Ka'Bandha. Seeing their Primarch fall at the hands of the Daemon caused despair and madness to overwhelm the Legion, but a few Blood Angels close at hand ensured Sanguinius was brought safely back to the Red Tear. Unfortunately, the Daemonic hordes unleashed on Signus Prime were intent on killing the Primarch and launched repeated assaults on the crashed flagship. Though they took a heavy toll on the Daemons, the 221st and their Brothers and allies were ground down with each attack. Soon, only a few defenders remained and, on the next attack, a flying Daemon grabbed Halbeth and took him into Signus Prime's skies. Even then, however, the Archein refused to stop fighting and struck repeatedly at any Daemons his weapon could reach. Halbeth would send several of them back to the Warp, before he was finally able to destroy the Daemon carrying him - but as a result, the Archein was sent plunging to his death.