After the Word Bearers had corrupted Calth's sun, Veridia, into giving off deadly radiation, the Ultramarines trapped on on the world sought refuge beneath its surface. Ankarion successfully led the remnants of the Legion's 547th Company to the Auroran arcology, which also sheltered two hundred thousand of Calth's population, including Pioneer Auxilia laborers. Shortly afterwards, a large force of Word Bearers attacked the arcology and Ankarion led its defense from the invaders. Unfortunately, the Word Bearers were able to corrupt the Consul-Chaplain into the worship of Nurgle and he aided them in destroying the 547th Company. Afterwards, Auroran was firmly in the grips of the Word Bearers who constructed a fane to Nurgle, which spread his corruption throughout the arcology. They also built icons to the Chaos God in the fane and one built of bone, allowed Daemons to possess the corpses of the 547th Company.
The Company's previous plea for aid, however, was received by the Ultramarines' central command on Calth. All contact had been lost with Ankarion', but as the 547th's request bore the Legion's highest priority, Captain Valius Thesian was sent to investigate what had happened to the archeology. Commanding the 22nd Chapter's 4th Destroyer Company, he entered Ankarion and his forces were quickly attacked by the Word Bearers and 547th's animated corpses. They succeeded in fighting their way to the former headquarters of the 547th and found Ankarion there, standing beside other Word Bearers. By then, the former Consul-Chaplain wore a suit of crimson power armor and his helmet was an indistinguishable mass of eyes, fanged mouths and curving horns. As Captain Thesian and the 4th Destroyer Company realized Ankarion had betrayed the Ultramarines, the corrupted Chaplain bellowed a challenge in ancient Colchisian and charged at them. The Word Bearers and the animated corpses of the 547th followed him, along with Daemons made from shadows.
However the 4th Destroyer Company charged at Ankarion's forces as well, as they had become enraged that a fellow Ultramarine was responsible for the Auroran arcology's corruption. Captain Valius Thesian and Ankarion then dueled each other, but the fallen Chaplain fatally wounded Thesian with an obsidian spear. As if sensing his approaching death, Warp Rifts began to appear next to Nurgle's fane and Plaguebearers began to force their way through them. With the last of his strength, though, the Captain activated a melta bomb, which consumed both him and Ankarion. The Chaplain's death caused the Warp Rifts to disappear and while the fighting continued, Thesian's 4th Destroyer Company was ultimately victorious.
- 1: Warhammer Community: Fight the Underground War with New Horus Heresy Missions and an Ultramarines Unit (Posted on 11/02/2022) - The Battle of Calth: Underworld War PDF, pgs. 3-5 (Last accessed on 11 February 2022)