In Iacon Destroyed of the Decepticon campaign, you'll come across a gallery room you must activate. A cinematic plays where Megatron converses with a holograph of Zeta Prime over the switch, if I recall.
At the center of this room is a very large model of Cybertron with planets circling it. After you've finished off all the Autobots, destroy every single black planet that is orbiting. Some are quite small, so be sure to get every one. Once you've hit every one, the achievement will pop.