This is how The Origins of Primus and Unicron goes in My Little Pony: The Revenge of Unicron.
Autobot X: Somewhere buried in this desert, the first two Cybertronians built a great machine. It harvests Energon by destroying suns.
Starlight Glimmer: Sounds quite supreme.
Trixie: Just like ruling Equestria with an amulet.
Thorax: An army.
Discord: And chaos.
Autobot X: You see, in the beginning, there were two Cybertronian brothers, the incarnation of creation and the incarnation of destruction. And they set out into the universe, seeking distant suns to harvest. The brothers set out with one rule: never destroy a planet with life. Until one of them tried to defy this rule. And his name, forevermore, was Unicron. [projects a hologram and narrates the events seen within] He despised the Equines, and he wanted to kill them all by turning on that machine. The only way to activate it is with a legendary key called the Dark Spark. A battle between brothers took place over the possession of the Dark Spark. Primus was stronger than his brother, then he had no choice but to steal and hide the Dark Spark from him. Somewhere, buried in this desert, that deadly machine reamins. Unicron knows where it is and if he finds the Dark Spark, your world will be no more.