The Brood
Upon migrating to the Milky Way galaxy, the Brood began to enslave the Acanti to take advantage of their natural ability to travel at speeds exceeding light. The Brood would infect individual Acanti with a disease known as the Slaver Virus, which would destroy the higher cognitive functions of their minds, making them ripe for easy control. Then, the Brood would transform the mindless husks into living starships. The Brood succeeded in enslaving the Acanti Prophet-Singer, and when he eventually died and crashed on a planet, its body was reconstructed as the "throne city" (presumably like a capital) on the planet (referred to as "Broodworld" or "Sleazeworld") that the Brood then established as the center of their empire. Thus, the Soul remained trapped in the skull of the dead Prophet-Singer.