Aeonid Thiel

Aeonid Thiel was an Ultramarines sergeant, later Captain of the Second Company, noted for his theory of killing Astartes and his part in the Battle of Calth. He began as a sergeant of the 135th Company and served alongside Roboute Guilliman during the Horus Heresy. Before the fighting at Calth, his theoretical studies addressed methods by which one Astartes might kill another, a subject the Legion’s genetically enhanced warriors were not expected to consider. The Legion regarded the work as a problem and censured him. The mark of that censure was a red helm, and he was posted to the flagship Macragge’s Honour, a Ultramarines vessel closely tied to the primarch’s own command.

At Calth, daemons boarded the flagship, and Thiel’s theoretical work became the only approach that Gage, the officer with whom he served aboard, was willing to heed. Thiel taught the ship’s defenders that predictability was the principal weakness to be corrected, and he led kill-teams through spaces tainted by the warp. He also stood with Guilliman on Kor Phaeron’s platform, placing him at one of the pivotal confrontations of the Ultramarines’ war against the traitor forces of Lorgar’s Word Bearers. Thereafter he took the Red-marked, those who bore the helm of censure, to the edges of Ultramar, so that the Codex Astartes would have a living argument in its favour. The Alpha Legion impersonated him in an attempt to shoot Guilliman; the real Thiel survived and returned.

By the end of the war Thiel held the rank of Captain of the Second, and he accompanied Guilliman onto Fulgrim’s Pride at Thessala. Among his contemporaries, the primarch Guilliman, the officer Gage and the Word Bearers’ Kor Phaeron are the figures most closely bound to his recorded deeds, with Macragge as his homeworld and Calth as the setting of his principal service. The red helm of censure has since become a sergeant’s honour, a lasting reversal of the judgement first passed on him. No account records Thiel’s fate after the teleportation aboard Fulgrim’s Pride, and his status is therefore given as missing.