"Darkness Rising" was the five-part mini-series that kicked off the Prime cartoon. As well as introducing us to the main Autobot, Decepticon and human characters, it set the tone early on by introducing and immediately killing off the Autobot Cliffjumper. Also playing a major role in the story was the return of Megatron with Dark Energon, the essence of Unicron, which has the ability to raise the dead as Terrorcons.
In Jasper, Nevada, three young humans - Jack Darby, Miko Nakadai, and Rafael Esquivel - are accidentally caught in the crossfire in a fight between enormous robots that transform into vehicles. They are taken to the base of the Autobots, where their leader Optimus Prime explains that he and his team (consisting of Ratchet, Bulkhead, Arcee, and Bumblebee) are autonomous alien robots from the planet Cybertron. The Autobots once had a devastating war against the tyrannical Megatron and his army, the Decepticons, and the war made Cybertron uninhabitable. Megatron returns from a three-year-long exile in deep space with a sample of Dark Energon, which can revive dead Transformers as mindless zombies called "Terrorcons", and he sets out to conquer Earth with a Terrorcon army brought over from Cybertron through a Space Bridge. The Autobots foil his plan by destroying the Decepticon's Space Bridge, and as Megatron is caught in the explosion he is presumed dead.