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'''Aeonid Thiel''' was a [[Space Marine Sergeant|Sergeant]] of the [[Ultramarines]] 13th [[Chapter]], 135th [[Space Marine Company|Company]], who distinguished himself during the [[Battle of Calth]] for his adaptability and unorthodox thinking.{{Fn|1a}}

It was customary for the [[Ultramarines]] to develop "theoreticals" for both personal and tactical combat, against all manner of [[xenos]] or human foes. Thiel, however, was marked for censure by his superiors for theorizing combats between [[Space Marines]]. His superiors felt that this scenario was both impossible and bordering on treasonous, but Thiel defended his reasoning, saying that Space Marines were the galaxy's most formidable warriors, and it was good sense to analyze their strengths and weaknesses as thoroughly as any other opponents'.{{Fn|1b1d}}

Thiel's superiors were not convinced, and he was sent to the flagship ''[[Macragge's Honour]]'' to be rebuked by [[Chapter Master]] [[Marius GageAntoli]], and possibly by [[Primarch]] [[Guilliman]] himself. While waiting for his superiors, he decided he was already in as much trouble as he could be, and spent his time practicing with the exotic melee weapons from Guilliman's private collection.{{Fn|1b}}{{Fn|1c}}

When the [[Word Bearers]] unleashed their traitorous attack, the ''Macragge's Honour'' was invaded by a horde of [[daemon]]s summoned from the [[Warp]]. Thiel quickly discovered that the daemons were more vulnerable to fire and melee weapons than traditional firearms. He fought his way to the bridge, where he saved Chapter Master Gage's life from another daemon.{{Fn|1b1d}}.

Thiel explained to Gage his thesis that, in a battle between former brothers, the Ultramarines' greatest weakness was their predictability, while the Word Bearers' greatest advantage was the shock of their betrayal; the only way for the Ultramarines to regain the initiative would be to dispense with both - using unorthodox tactics and strategy and wage war against their former brethren with all ferocity. Gage quickly approved of Thiel's logic, and detailed him to lead the parties of [[Ultramarines]] to scour the remaining daemons from the ship.{{Fn|1b1d}}

Thiel was later one of the party of fifty Ultramarines who accompanied Guilliman aboard the orbital weapons platform occupied by [[Kor Phaeron]] and his elite guard, the [[Gal Vorbak]]. {{Fn|1f}} Thiel fought bravely, but was unable to prevent Kor Phaeron escaping.{{Fn|1g}}{{Fn|1h}}

At the time of the [[Great Crusade]], it was customary for Ultramarines marked out for censure to bear red markings on their helmets. During the Battle of Calth, Guilliman ordered that the squad leaders accompanying him aboard the orbital platform paint their helmets red; because the Word Bearers' attack had shredded the Ultramarines' normal command hierarchy and unit organization, and because [[vox]] communication was unreliable, Guilliman wanted clear, visual cues for the Ultramarines to follow. Furthermore, he added that after Thiel's heroism of the day, red markings would be badges of honour, instead of censure.{{Fn|1c1e}}

**{{Endn|1a}}: ''Dramatis Personae''**{{Endn|1b}}: Target//Acquisition, Chapter Four, pgs. 51-53**{{Endn|1c}}: Target//Acquisition, Chapter Eight, pgs. 93-95**{{Endn|1d}}: Target//Engagement: Chapter Four, pgs. 239243-48**{{Endn|1c1e}}: Ushkul/Thu: Chapter 2Two, pg. 351**{{Endn|1f}}: Ruin//Storm: Chapter One, pg. 368**{{Endn|1g}}: Ruin//Storm: Chapter Three, pgs. 384-385**{{Endn|1h}}: Ruin//Storm: Chapter Four, pgs. 392-393, 396