Corswain was a Paladin (Company Champion) of the 9th Order of the Dark Angels, a member of Lion El'Jonson's personal Honour Guard, during the Horus Heresy. A warrior of great skill and renown in the Legiones Astartes of old, his name could be counted alongside such illustrious warriors as First Captains Jago Sevatarion of the Night Lords, Ezekyle Abaddon of the Sons of Horus, Chapter Master Raldoron of the Blood Angels and Lord Commander Eidolon of the Emperor's Children.
During the opening years of the galaxy-wide conflict known as the Horus Heresy, Corswain took part in the brutal campaign called the Thramas Crusade in the Eastern Fringe of the Ultima Segmentum. Two standard years into the campaign, the Night Lords' Primarch Konrad Curze attempted a parley upon the planet of Tsagualsa. Corswain accompanied Lion El'Jonson alongside his fellow Honour Guard member, Master Alajos. Curze was also accompanied by two members of his Honour Guard, First Captain Jago Sevatarion (Sevatar), and Captain Sheng. During the parley the conversation between Primarchs started amicably enough, until Curze slandered El'Jonson. In return, El'Jonson struck his former brother and a fight ensued. The Night Haunter eventually gained the upper hand during the fight and began strangle the Lion to death, but El'Jonson was rescued by the timely intervention of Corswain when the Paladin ran his sword into Curze's back, rendering him comatose. During the fight, Sevatar decapitated Master Alajos, and Sheng helped his comatose Primarch escape when reinforcements from both Legions arrived. Following this incident, Corswain's fate during the Horus Heresy remains unknown.
- Age of Darkness (Anthology), "Savage Weapons," (Short Story) by Aaron Dembski-Bowden
- The Primarchs (Anthology) "The Lion," (Short Story) by Gav Thorpe