During the opening years of the galaxy-wide conflict, known as the Horus Heresy, Corswain took part in the brutal campaign known as Thramas Crusade in the Eastern Fringe of the Ultima Segmentum. Two 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 had begun to strangle