Guillaume was the Imperial Guard Captain of the Cadian 92nd Regiment, who was charged with protecting an Imperium city from the advancing Waaagh! of Warboss Nazgob. Compounding his duty, was the presence of Inquisitor Garric in the city and the need to evacuate him to safety, via Valkyrie, as the Inquisitor possessed needed vital information. As the Waaagh! charged the city, Guillaume gave the order for his troops to fire, but it was not enough to prevent the Orks from the reaching the Regiment's frontlines. In the close combat that followed, Guillaume fought Nazgob, but was torn in half by the Warboss' power claw. Despite his death however, the Cadian 92nd fought on and not only managed to evacuate Inquisitor Garric, but also saw to the Warboss' death; at the hands of the Bullgryn Rogg. With the Warboss dead, the Regiment was able to defeat the Waaagh! and save the city.