Unicron (Generation One)

Unicron is the main antagonist of The Transformers: The Movie, and a minor antagonist of season 3. Unicron is an all powerful being, able to transform from a planet into a enormous robotic body, Unicron wishes to consume all life in the universe.

History
Backstory

In the season 3 episode, Call of the Primitives, Unicron was constructed by the ancient genius known as Primacron at the dawn of the universe. Primacron had hoped that Unicron would scour the galaxy clean of life, leaving a blank slate for him to experiment upon, but naturally, Unicron had no desire to be the servant of another, and turned on Primacron, badly wounding him and his assistant, and leaving him.

The Movie
In the year 2005, Unicron made his attack upon Cybertron. While heading towards the planet, he consumed the small world of Lithone. As he continued on his way, Unicron's powers allowed him to become aware of the death of Optimus Prime, who handed the Matrix of Leadership to Ultra Magnus just before dying, which angered him. He then came upon the bodies of several wounded and dying Decepticons, who had been dumped into space following a recent battle on Earth. He extended a proposition to the Decepticons' leader, Megatron, offering to rebuild his body and those of his troops in exchange for their destruction of the Autobot Matrix of Leadership. Tortured into accepting, Megatron was transformed into Galvatron, and his followers became Cyclonus, Scourge, and the Sweeps.

While Galvatron reclaimed leadership of the Decepticons, Unicron arrived at Cybertron, and proceeded to consume the planet's two moons. Outraged that Unicron would do this to a planet that he viewed as his, Galvatron found himself the victim of mental torture by his creator, and ordered to complete the task given him. Galvatron seized the Matrix and returned to Unicron, only to reveal that he intended to use the talisman to make Unicron his slave. Amused by his pawn's arrogance, Unicron transformed into his gigantic robot mode and ate Galvatron. Unicron then tore into Cybertron, as Shockwave and the rest of the Decepticons on the planet fought back, to little effect. Unicron swallowed many spaceships, and one, which contained the young Autobot Hot Rod and his comrades, crashed through Unicron's eye. Hot Rod and Galvatron battled deep inside Unicron, and ultimately, Hot Rod was able to retake the Matrix from the Decepticon. Transformed by its power into Rodimus Prime, he defeated Galvatron and opened the Matrix inside Unicron, destroying his body from within. Unicron's head was blown clear during his death throes, and became Cybertron's new moon.

Season 3

A year later, in Five Faces of Darkness, Cyclonus, Scourge and the Sweeps visited Unicron's head, seeking to locate Galvatron, who had been hurled into space at the conclusion of his battle with Rodimus. After accessing the remaining databanks and replaying footage of the final battle, Cyclonus extrapolated Galvatron's trajectory and current location.

In Ghost in the Machine, the ghost of the deceased Decepticon Starscream reactivated Unicron's head in order to make a bargain with him; in exchange for Unicron restoring his body, Starscream would recreate Unicron's. Stealing new optic circuits and a transformation cog for Unicron from Metroplex and Trypticon, Starscream was then directed to connect Unicron's head to Cybertron, so that the planet could become his new body. Starscream initially attempted to do this by using Trypticon's body, but Trypticon was deactivated by the Decepticons. Unicron restored Starscream's body so that he could finish the task, but Starscream double-crossed Unicron. Just then, explosives set by the Autobots were detonated, hurling Unicron's head off the planet, beyond its gravity, and out into space.

In Grimlock's New Brain due to seeking a supply of anti-electrons, Galvatron and the Decepticons traveled to Unicron's head, only to come under attack from his defenses. Also present was the Dinobot, Grimlock, who used components from Unicron's head to create the Technobots to fight Galvatron's forces. The Technobot Strafe disconnected enough of Unicron's internal mechanisms to deactivate his systems, apparently killing the evil giant.

In Call of the Primitives, a much older Primacron mused about how his new creation, Tornedron, would succeed where "the other" had failed, the giant, damaged head of Unicron was visible floating along in the depths of space, where it had apparently faded to a dull grey.

Trivia

 * Unicron's origins differ in the Generation One cartoon in comparison to most other media, which take inspiration from the Marvel Comics and depict Unicron as a dark God.