Unicron
The arch-enemy of his brother Primus, Unicron is dedicated to consuming the multiverse. The only thing that he fears is Primus' essence, contained inside the Matrix of Leadership.
In the beginning
At the birth of the Universe, an extradimensional entity known as The One created twin astral beings to reign over the cosmos. One was Primus, the Lord of Light and Order. The other was Unicron, the Lord of Darkness and Chaos. Unicron sought to devour everything that exists and become one with The Void at the end of all things.
Primus was no match for Unicron. But Primus did manage to trap both his and Unicron's essences inside metallic planetoids. Over time, Unicron learned to shape his planetoid into a metal being that was able to transform between planet mode and a robot mode which resembled his original astral form. Primus hid himself in slumber, and their war ended, for a time, in stalemate.
The War of the Balance
Grimlock, Jazz, and Bumblebee found themselves transported to the center of Cybertron. In a battle with Bludgeon's Pretenders, a laser blast ricocheted off Grimlock and struck the essence of the sleeping Primus, causing a multiverse-shaking Primal Scream that woke Unicron and alerted him to Primus' location.
The battle between Primus and Unicron began anew. Though Unicron would sometimes be defeated, this is always temporary, and there is a whole multiverse of realities to consume. When Unicron is destroyed in one reality, he will respawn in another, ready to eat again.
