Nemesis (ネメシス, Nemeshisu) the Decepticons' fortress of a warship, that thes left Cybertron on during the Great Exodus in the middle of the Great War. Carrying Megatron and the remnants of his Decepticon uprising, the Nemesis was a force to be reckoned with until it suffered many damages during the battle against the Ark.
The Decepticons have been designing the Nemesis during before Optimus became a Prime, knowing that Cybertron would suffer casualties and probably halt its energon-giving sources if Megatron or the Primes countinued to esculate the war any longer with either of them continue leading the Autobot and Decepticon forces. Since there are other Autobots and Decepticons that would chose to stay behind, Megatron had chosen Shockwave to take his place in leading the Decepticons in his place.
When Shockwave have located the AllSpark on a primitive planet rich with raw energy, Megatron have chosen enough Decepticons to come with him aboard the Nemesis, leaving Shockwave in charge of the Decepticon forces on Cybertron. The Nemesis, pursued the Ark as it departed Cybertron, and engaged it in a fierce ship-to-ship battle in space. After Optimus and Megatron dueling from the colliding of their weapons, a rip in space-time began tearing both ships apart, and ultimately sucked both sides and ships to parts unknown.