At the predawn of time when the universe was fresh and new the All Spark sent two powerful beings out to see how the cosmos was fairing -- these two brothers were Primus and Unicron. Charged with exploring the vastness of space they traveled from one end of the universe to the other. They discovered a region of space spewing forth pure Energon, power beyond their infinite dreams. Primus could not wait to tell their father what they had found. But Unicron had other plans; a great desire welled up in his soul -- he had to have it for himself. When Primus wasn't looking unicron smote his brother and then cast his seemingly lifeless body into a nearby star cluster. And thus began the transformation of Unicron into the dark god, the black sinister side of the spark. Unicron is a massive moon-sized battle fortress brimming with thousands of gun emplacements and a gargantuan maw he uses to devour worlds. His soul burns with an all-consuming thirst, a craving that cannot be sated. He ultimately dreams to rule the universe one day, and to dominate and control all Transformers.
Bio and image taken from the 2003 Fleer Transformers Armada Trading Card set.