Nemesis Prime

Decepticons
Sideways was the best friend of Sideswipe. The two fought together being in the Elite Guard, but when the Decepticons rose, the two friends split sides. Sideways became loyal to Megatron and kept fighting to destroy the Autobots.

When the war kept on going, Sideways became a true killer. But when he was sent on a mission with Sunstorm and Cyclone they ran into the giant Unicron. They were then remade by Unicron. Sideways became Sidelock while Cyclone and Sunstorm became Cyclonus and Scourge.

Terrorcon
Sidelock kept fighting against the Autobots and Decepticons. But as time went on, the Autobots fought Sidelock and managed to kill him. This made Unicron give Sidelock's body the Dark Matrix. The Dark Matrix made Sidelock into the opposite of a Prime. He became Nemesis Prime.

Post War on Earth
Years later, Nemesis Prime returned to destroy Cybertron. He used Dark Energon to infect the planet. Doing this, Vector Prime traveled back in time to stop Nemesis Prime. Optimus Prime also helped Vector Prime and together they were able to defeat Nemesis Prime. But this ended with Optimus dying as well.

This wasn't the end of Nemesis Prime though. He was recreated years later when Galvatron brought Unicron to Cybertron. Nemesis Prime now returned but tried to betray Galvatron. This made Galvatron rip the Dark Matrix out of Nemesis Prime's chest. This killed Nemesis Prime for good as he had no spark after his first death.