As Unicron and Primus venture through the cosmos, it becomes apparent to Primus that Unicron is nothing but a purely corrupt being, and so confronts the threat his sibling poses. In combat, Primus is no match for Unicron; in cunning, he proves himself to be his brother's superior when he shifts their battle to the astral plane, and then back to the physical world, sacrificing most of his strength to manifest their essences within metallic planetoids and leaving them both trapped in the planet prison. Primus hopes to contain the evil force forever. Unfortunately, over time, Unicron learns to physically shape his prison into a metallic planet, and Primus emulates his brother. The Primus planet becomes the mechanical world of Cybertron. When Unicron learns to transform his planetary form even further, into a gigantic robot form, Primus adapts the idea, and creates The Thirteen, a group of robotic beings that possessed the ability to change shape.