The Energon Pub

The Prime Masters are the small figures that the line are sorta built around. They can stand in Pretender shells, ride in/on Legends, and attach to the armor of deluxe figures. Oh, they can also be used as heads, on Titans Return figures and interact with Titan Master drones and bases. One negative is that a lot of the uniqueness of a Titan Master is lost with Prime Masters, and the Pretender shells transform the same way thus far. For five bucks USD though, you really can't go wrong.