Page "Primus (Transformers)" Paragraph 26

As Unicron and Primus went about their appointed task, venturing through the cosmos, it became apparent to Primus that Unicron was a corrupt being, and he took it upon himself to stop the threat posed to all of existence by his sibling.

In combat, Primus was no match for Unicron.

In cunning, however, he proved himself to his brother's superior when he shifted their battle to the astral plane, and then back to the physical world once more, only to have both their essences manifest within metallic planetoids, leaving them both trapped.

It was with this act of sacrifice that Primus hoped to contain Unicron's evil forever.

Unfortunately for him, over time, Unicron learned to psionically shape his prison into a giant metallic planet, and Primus followed suit, becoming the mechanical world of Cybertron.

When Unicron then learned to transform his planetary form even further, into a gigantic robot form, Primus adapted the idea to suit his own ends — rather than transforming his own body, he would create small beings that would be able to change their shape, like Unicron.

After performing a " test run " on the moon of Protos, where he successfully created twelve transforming robotic beings, he birthed from his own body a group of thirteen robots that possessed the ability to change shape, like Unicron.

These were the thirteen original Transformers, each one infused with a fragment of Primus's life essence known as a spark.