The Bestiarius was an Frigate in the World Eaters Legion, during the Horus Heresy and was among its fleet that journeyed to take part in the invasion of Terra. By that time, their flagship, the Conqueror, had fallen under the influence of Khorne and had become a hostile environment to those aboard it. This led to the rapid depletion of supplies and manpower - chiefly due to the World Eaters losing themselves to mindless rages and then attacking anyone near them. In response to this, a group of World Eaters and crew members secretly met and, in order to save their lives, began plotting to escape the cursed flagship. Led by the Conqueror's own Captain, Lotara Sarrin and the Destroyer Sergeant Skane, the group planned on using shuttles to escape to the Bestiarius the next time the World Eaters' fleet exited the Warp. However, when their plan was under way, Sarrin betrayed the group to their Legion, as she had never planned on abandoning her command post, and she then ordered the Conqueror to destroy the escapees' shuttles.