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. Consul-Chaplain 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. The Company then sent a request for immediate aid, but due to the incursions by the World Bearers, who also sought to escape Veridian's radiation, the Ultramarines' central command on Calth lost contact with Auroran. As their request bore the Legion's highest priority, 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 the 547th's animated corpses. Dorian and his fellow Dreadnought Andronicus had been attached to the 4th Destroyer Company during this battle and their firepower greatly aided in killing numerous foes. Thanks to the two's efforts, the Company succeeded in fighting their way to the former headquarters of the 547th, which had been turned into a fane to the Chaos God Nurgle. They also found Ankarion there, standing besides a group of Word Bearers and Chaos icons. But the Consul-Chaplain now wore a suit of crimson power armor and his helmet was an indistinguishable mass of eyes, fanged mouths and curving horns. As Dorian 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. With the last of his strength, though, the Captain activated a melta bomb, which consumed both him and Ankarion. The fighting between their surviving forces continued, but Andronicus began a wild rampage after Dorian was killed during the battle. The enraged Contemptor Dreadnought tore apart multiple foes and also destroyed a Chaos icon within the fane. Its destruction, though, put an end to the the 547th's animated corpses and allowed the 4th Destroyer Company to defeat the remaining Word Bearers.
- 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)