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

*{{Main|Battle of Calth}}

{{Spoiler|source=Know No Fear (Novel)}}''

[[Image:Censure-large.jpg|thumb|right|220px|Sergeant Thiel on the surface of [[Calth]].]]

{{Spoiler|source=Unremembered Empire (Novel)}}''

Later, Thiel sought an audience with Guilliman, hoping to brief him on the situation on Calth. Guilliman eagerly agreed, for he valued Thiel's honesty and directly-to-the-point attitude. During their audience, Thiel asked if Guilliman would mind meeting his squad, all 'orphans' from previously destroyed squads on Calth who had banded together under Thiel. Again, Guilliman agreed. This would almost cost the Primarch his life as Thiel and his men revealed themselves to be [[Alpha Legion]] assassins and shot Guilliman at point blank range with their [[bolters]], though the Primarch managed to kill of of them before collapsing. A subsequent investigation was unable to determine if this had been the 'official' Thiel and his squad, or if the assassins had simply assumed their identities on route to Macragge.

{{Spoiler|source=Stratagem (Audio Drama)}}''

{{Spoiler|source=Nightfane (Audio Drama)}}''

Thiel was later part of Guilliman's fleet of reinforcements as it rushed to rescue the loyalists in the [[Siege of Terra]].{{Fn|8}}

Aeonid Thiel survived the [[Horus Heresy]], becoming the [[Space Marine Captain|Captain]] of the [[2nd Company (Ultramarines)|2nd Company]] of the Ultramarines [[Chapter]]. He took part in the [[Battle of Thessala]], during which Roboute Guilliman was mortally wounded by [[Fulgrim]].{{Fn|6}}

At the time of the [[Great Crusade]], it was customary for Ultramarines marked out for censure to bear red markings on their helmets.{{Fn|1c}} 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|1e}}

Immediately after the Battle of Calth, it became customary for [[Sergeant (Space Marines)|Sergeant]]s of the Ultramarines, to paint their helmets red to signify their rank, including those of the force led by [[Remus Ventanus|Captain Ventanus]] during the [[Underworld War]].{{Fn|2}} The tradition has endured in the ten thousand years since the Heresy, among the Ultramarines and several [[Codex Chapter]]s.{{Fn|5}}

{{Cite}}

*1: [[Know No Fear (Novel)]]: