Transformers: Earthrise features a time-displaced meeting between Megatron and Galvatron, setting the stage for Unicron in the final chapter of Transformers: War for Cybertron. A darker, grittier take on the Transformers franchise, War for Cybertron's second chapter, Earthrise, finds Megatron in hot pursuit of the Autobots aboard the Ark as they attempt to locate the life-giving artifact known as the Allspark.
Along the way, the Autobots and Decepticons find themselves trapped in an alternate dimension called the Dead Universe. Once inside the Dead Universe, Optimus Prime is guided out by Sky Lynx, a humbled Autobot who has been trapped in the Dead Universe for years. Megatron is guided by an entirely different kind of herald - Galvatron, who he is destined to become. Somehow utilizing the Dead Universe's tenuous relationship with time and reality, Galvatron is able to time travel to tell his past self about the Golden Disk, an artifact that originated on Earth, and which could be the key to the Decepticons winning the war. Before he can fully explain his plan, Galvatron is pulled back to his own time, pleading with Megatron to prevent his possible future as a slave to Unicron.
In the classic 1986 animated movie, The Transformers: The Movie, Optimus Prime and Megatron both fall in the Battle of Autobot City. Optimus Prime succumbs to his injuries, passing the Autobot Matrix of Leadership onto his trusted lieutenant Ultra Magnus. The badly damaged Megatron, meanwhile, is recovered by his soldiers, only to be jettisoned into the vacuum of space by Starscream. Megatron and a handful of other fatally injured Decepticons make their way to Unicron, the massive, planet-sized Transformer.
In an effort to destroy the Matrix, Unicron makes a deal with Megatron - he'll give him a new body and new troops to command to meet his nefarious goal. Facing death square in the face, Megatron reluctantly agrees, and is reformatted into Galvatron, a much more powerful version of Megatron. The tradeoff is that he's a slave to Unicron, unable to pursue universal conquest like he feels he's destined to.
Galvatron's appearance in the Dead Universe would seem to suggest Unicron is just around the corner for Transformers: War for Cybertron considering Earthrise's Easter eggs. It's unclear how Unicron will figure into the final chapter of the trilogy, Transformers: Kingdom, but that series promises all sorts of time travel shenanigans as the heroes of Beast Wars meet their Generation 1 counterparts if the events of Transformers: Earthrise are any indication. That's good news, because it may take two generations of heroic Cybertronians - as well as a version of Megatron with inside information - to stop Unicron.