Unicron

''"An urban legend from the olden days tell the story of Cybertron's ancient enemy... Unicron... the Destroyer of Worlds sought only to bring destruction and chaos everywhere he went. Apparently, he was defeated millions of years ago by Primus. But it's just a legend. No one believes he actually exists..." ''—Shockdrop tells Tessa about Unicron

The ancient enemy of Cybertron, Unicron is the Bringer of Chaos and a Destroyer of Worlds. According to legend, Unicron was defeated millions of years ago by Primus (the original form of Cybertron). As a result Unicron was merely a myth among the Transformers; neither Autobots nor Decepticons actually believed he existed. Apparently, Moltenstorm holds allegiance to the Chaos Bringer for reasons unknown. While the Autobots and Decepticons were preoccupied on Earth, Moltenstorm and his forces got to work restoring Unicron on the planet's moon. Unicron was part of Moltenstorm's plan to destroy both Earth and Cybertron -- this news somehow reached Quintessa, who desired to stop Unicron's rise to power. Moltenstorm was partially successful in restoring Unicron's motor functions: allowing the Chaos Bringer to speak and observe the Transformers on Earth.

Before construction could be completed, the Autobots launched an attack on the moon (supplied by The Agent for their attack). Unicron assumed control of the Predacon forces, using Moltenstorm to combat the Autobot threat. Under Optimus Prime's command, however, Moltenstorm was defeated by the Autobots and the construction site left in smoldering ruins. As a result of his failure, Unicron used his telepathic abilities to terminate Moltenstorm and assumed full command over the remaining Predacon forces.

Because of his actions, Unicron may very well be one of the primary antagonists of Transformers Robot Defenders.