Nemesis Prime is the name of several fictional characters from the Transformers storyline. He is often depicted as an evil incarnation of Optimus Prime.
In "puppet" in the Armada universe, Nemesis Prime (Scourge in the Japanese version) was a monstrous Transformer, controlled by Sideways, sent to kill the Autobots and Decepticons while they were on their way to Cybertron. It scanned the minds of the Transformers that landed on the planet it was waiting on to ambush them and took the form of Optimus Prime as he was the most powerful and respected in all their minds. As a duplicate of Optimus Prime, Nemesis had all of Prime's strength and power - in addition to his ability to reconstruct himself. Nemesis Prime subsequently attacked and decimated the two sides, leaving most of the Transformers heavily damaged. However, they were then restored and their abilities enhanced by the powers of their Mini-Con, and Nemesis Prime was subsequently destroyed by the power of Optimus Prime (powerlinked with Overload) and Galvatron, the newly reformatted form of Megatron.
Nemesis Prime wielded the Dark Saber, a twisted clone of the Star Saber, and had a Mini-Con partner named Run-Over. Run-Over was only seen alongside Nemesis Prime in the Linkage DVD comics released in Japan. Here, he was used in gun mode, to help Nemesis Prime create an artifact called the "Doomstone," made of the death agonies of their victims. The Doomstone is an artifact of immense power, capable of destroying a soul-dimension such as the Linkage. If destroyed, the energy released could also wipe out all life on a planet.
In the anime, Nemesis Prime had a trailer not unlike Optimus Prime's in terms of design and could presumably transform to super mode with it. But the toy came without a trailer and contained none of the circuitry required to transform a trailer.
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