Primus was a deity that featured in the Transformers universe.
Primus was the god of light, created as an enemy to Unicron, the Chaos-Bringer. Their battle raged, Unicron seeking to destroy the newborn universe and Primus to protect it. They fought across the universe and into planes of the mind, until Primus tricked Unicron, trapping both their essences in dead asteroids.
Unicron eventually reshaped his into the planet-eating terror he became known as, while Primus took a different path. He made his asteroid a world of living machines- Cybertron- and bred a race of living machines- Transformers- to serve as the warriors to finally destroy Unicron. To help their numbers grow, he gave them the ability to reproduce via bio-morphic reproduction, until he decided there were enough of them and wiped the ability from their memory. (Despite this, later Transformers, including Lord Jhiaxus, rediscovered the ability and used it to excess.) He gave them a large portion of his life energy in the form of the Creation Matrix, allowing them to create new Transformers using the artifact. After that, Primus went into hibernation, with only the Keeper to guard him.
However, in 1990, Primus was reawakened, unleashing a scream that alerted Unicron to his location. Unicron began traveling to Cybertron to destroy his enemy, and sent Galvatron, Hook, Line, and Sinker to soften up the planet's defenses. Galvatron betrayed Unicron, however, and with the help of Emirate Xaaron, he awoke Primus again, who inhabited Xaaron's body. Primus summoned all the Transformers to Cybertron, but in trying to muster them to fight the overwhelmingly terrifying Unicron, Primus (along with Xaaron) was destroyed.
Primus has been mentioned, but little is known about him.
According to the Transformers Ultimate Guide, Primus was capable of transforming Cybertron into a robot like Unicron, but never chose to do so.