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 consuming other planets. Death's Head claimed to have battled him on the psychic plain 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)