Unicron is a prodigiously large robot whose scale reaches planetary proportions; he also able to transform into a giant planet.

A planet-mode diameter of 1,000 km (620 mi) has been suggested[6] and robot-mode dimensions can follow from that estimate, provided that there is no change in size (compression or expansion) common among so many Transformers.

[8][9] The character Unicron first appears in the opening scene of 1986's The Transformers: The Movie, immediately making clear his goal by consuming the small world of Lithone.

Attempting to open the Matrix to intimidate Unicron, Galvatron only prompts his transformation into robot mode and is swallowed by the planet-eater.

Unicron lays waste to Cybertron, halted briefly when the young Autobot rookie, Hot Rod, crashes a Quintesson spacecraft through his eye.

[13] The third season of The Transformers animated series continues Unicron's story where the movie left off, as the planet-eater's deactivated head settles into orbit around Cybertron.

[14] Later, the ghost of the deceased Decepticon Starscream reactivates Unicron's head and enters into a bargain with him, performing three labors in exchange for the restoration of his body.

[17] Later, when searching for a new supply of positrons (anti-electrons), the Decepticons venture to Unicron's head, where Cyclonus and Scourge accidentally awake the slumbering demi-god.

In episode 23, several of the human allies discover Sideways within their computers, and attempt to stop him, but not before learning that his accomplice was Unicron within the mainframe acting as a failsafe.

[24] When Unicron attacks Cybertron in the midst of the chaos caused by a Mini-Con civil war, Sentinel Maximus and Omega Prime fight him off with the help of their allies.

To accomplish this, Unicron opens dimensional rifts in order to bring Curse Armada Thrust, Zombie War Breakdown, and Wandering Roller into their world.

[26] Debuting in Transformers: The Last Knight, the six horns of Unicron began appearing out of the Earth in response to the approach of Cybertron.

Quintessa plans to drain Unicron's energy to restore the damaged Cybertron, killing life on Earth in the process.

Devouring the home planet of the Maximals, who escape with the Transwarp Key to Earth, he sends his Terrorcon servants to find it.

[29] Unicron begins attacking Cybertron, requiring the Autobots to collect enough Mini-cons to create a power-up mode for Optimus Prime.