Signature
A prequel prequel. Megatron rises to power from Kaon's underworld, Ratbat falls from the glory of the Golden Age, and Soundwave, a drone, learns of what part he is to play in the destiny of Cybertron.
(This expands upon the brief histories given in both Transformers: Prime and also War For Cybertron, setting the stage for the events that will come later and detailing the reasoning behind some of both universes' unique features: Drones, Megatron's engineering skills, the origins of mechanical life, etc)
Many cameos await.