A notable warrior, Luc Sedirae's martial exploits were considered to be not far behind those of First Captain Ezekyle Abaddon.1
Sedirae was considered at one point for elevation into the Mournival brotherhood, but was passed over in favour of Garviel Loken. The rumoured reason for this was that he was considered too ruthless in battle and too eager an advocate of warfare. The Primarch Rogal Dorn talked of Luc Sedirae as a battle-hungry killer ready to say yes to any plan that would lead to glory in combat.1 It is likely therefore, that his opinion would have unbalanced the Mournival's council and was therefore deemed unsuitable.
Despite his fearsome reputation as a combatant, in person Luc Sedirae was seen as charming, friendly, charismatic and handsome. Blonde-haired and blue-eyed, he always seemed to be smiling, although sometimes his habit of keeping his mouth half-open gave the impression that he was just about to bite something. Sedirae often deployed himself in an assault role, and therefore wore a jump pack upon occasion.2
Sedirae took part in many notable campaigns during the Great Crusade. On the world of Murder, he achieved glory in leading his company against a force of Megarachnid over 2,500 strong; 13th Company shattered the Megarachnid force in half an hour.1 He fought the unnatural forces of 'Nurg-leth' on the moon of Davin, saving Garviel Loken at one point and leading the assault on the Glory of Terra at another. During the assault on the downed starship Sedirae and his cohort were cut off from their fellows and found themselves in close combat with Nurgle daemons. This experience seemed to unnerve the Sons of Horus Captain.2
A firm believer in the warrior-lodge and a companion to Serghar Targost, Luc Sedirae willingly followed the direction of his Primarch and co-commanded the initial Sons of Horus deployments against the loyalist marines upon Isstvan III.3
Sedirae was later present on the planet Dagonet in the Segmentum Ultima. The planet had been the site of one of Horus's first conquests, in which he claimed the planet for the Imperium with a single bolter shot. After a lengthy civil war seemed to have died down, with the pro-Horus aristocracy emerging on top, Dagonet declared for the Warmaster. To commemorate the event, Horus came to the system to attend a ceremony in which the planet welcomed Horus as its twice-come liberator from oppression. However, it was Sedirae who descended to the planet in Horus's place, acting as his body-double and wearing the Warmaster's regalia. This choice saved Horus's life, as upon removing his helm, a team of assassins led by the Vindicare sniper Eristede Kell managed to kill the unsuspecting Sedirae. Angered, the Sons of Horus killed the entire aristocracy in a matter of moments, and then turned its aggression against the entirety of the planet, leaving none alive.4