Unicron

Unicron was a vast mechanoid and Transformer (COMIC: Meet Death's Head) that the Death's Head Interactive Archive later also identified as a planet. (COMIC: The Incomplete Death's Head) Unicron was known for devouring other planets. Death's Head claimed to have battled him on the psychic plane and to have personally defeated him. (COMIC: Meet Death's Head) However, in doing so, Death's Head was caught in Unicron's gravitational well and accelerated to trans-temporal speeds, throwing him into the Space-time continuum. This allegedly freed Head from the Transformers universe, and threw him into the Crossroads of Time. (COMIC: The Incomplete Death's Head)

There, Head collided with the TARDIS, meeting the Seventh Doctor. Head told the Doctor that he had been "thrown into the time vortex during altercation with [his] last target. He died." (COMIC: The Crossroads of Time) The Death's Head Interactive Archive believed that the battle which had resulted in Death's Head leaving the Transformers universe was one of the final engagements in the Transformer war. (COMIC: The Incomplete Death's Head)

Behind the scenes
The depiction in The Incomplete Death's Head of Death's Head being accelerated to trans-temporal speeds due to being caught in the gravity of "a collapsing planet" is not a totally accurate recap of the events of The Transformers comic ''[https://tfwiki.net/wiki/The_Legacy_of_Unicron! The Legacy of Unicron!]'' In the actual story, Death's Head chooses to open a time portal to escape the destruction of Unicron, saving Cyclonus and Scourge in the process.

Unicron is not directly named in any of the stories covered by Tardis Wiki where he is mentioned. In Meet Death's Head he is called "a vast planet devouring mechanoid," in The Crossroads of Time he is referenced as Death's Head's last target who sent him into the time vortex, and in The Incomplete Death's Head he is simply called a collapsing planet whose gravitational pull threw Head into the space-time continuum. However, considering that Meet Death's Head is a crossover of sorts, this wiki's policy deems that pulling Unicron's name from Transformers source material is not conjecture.

G1 Unicron was voiced by Orson Welles in his final film role in 1986's The Transformers: The Movie.