Information from The Incomplete Death's Head and its respective stories needs to be added.
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Death's Head was a metallic life-form who worked as a "Freelance Peacekeeping Agent" (effectively, a mercenary, bounty hunter and assassin, though he hated being called any of those things) in a number of dimensions and time zones. Death's Head described himself as a mechanoid, a designation not related to the Mechonoids, an enemy of the Daleks.
Death's Head did not take pleasure in killing, merely in being professional and financially astute, and possessed a rather pedantic personality. He was a habitual traveller between alternate realities, spending most of his career in universes other than the main one. At the time that he first encountered the Seventh Doctor, Death's Head had been enlarged to nine metres in height.
Death's Head I
Death's Head was created as a cyborg body in a realm called Styrakos by Lupex and Pyra. An unknown party stole the body, programmed it with a “killer instinct” and dumped it on Scarvix, (COMIC: The Incomplete Death's Head) where Death's Head made a living as a bounty hunter. (COMIC: Death's Head!)
After some months, the Doctor transplanted Death's Head through a warp gate to a parallel universe of massive warring robots, where he was subjected to major restructuring surgery and replacement of living metal components, resulting in him being greatly enlarged. During the final battles of the war, he was caught in the gravitational well of a collapsing planet and flung into the Time Vortex. (COMIC: The Incomplete Death's Head)
While travelling through the Time Vortex, Death's Head collided with the Doctor's TARDIS, forcing both to materialise. At first, Death's Head intended to kill the Seventh Doctor, but was shot with a Tissue Compression Eliminator and returned to his original size apparently uninjured. Wishing to return to his own time, Death's Head let the Doctor lure himself into the TARDIS, from where the Doctor sent the mechanoid through time and space to the planet Earth in the year 8162. (COMIC: The Crossroads of Time)
After an encounter with Dragon's Claws (COMIC: Watch Out – Dragon's Claws Here's Death's Head!), Death's Head was repaired by Spratt. During this he remembered his time on Scarvix. (COMIC: Death's Head Revisited) He then helped the Chain Gang attack Dragon's Claws. (COMIC: Contractual Obligations)
Death's Head later attempted to claim the price put on the Doctor's head by Josiah W. Dogbolter, using a prototype time machine to track him down. However, Dogbolter meant to double-cross the mechanoid, hiding a nuclear device in the time travel unit to kill both him and the Doctor. The Doctor saved Death's Head from the bomb and in return the mechanoid decided not to complete the contract on the Time Lord, this time. (COMIC: Time Bomb!)
On a different occasion, Death's Head, while counting money, raised his glass in acknowledgement to the Doctor at Bonjaxx's party. When a drunk Beep the Meep started a fight, Death's Head happily joined in. (COMIC: Party Animals) On the way out of the party, Death's Head was attacked by Dogbolter's old servant Hob but was saved by teaming up with his own future self. He attempted to kill his future self, but the Doctor stopped him with a modified TCE. The Doctor erased the memory of this encounter from Death's Head and returned him to the party. (COMIC: The End... Yes?)
Death's Head II 
At some point, a cyborg from the year 2020 programmed with 105 "personalities" by Doctor Necker killed Death's Head and assimilated his memories, rather than just his personality. This allowed the cyborg, now known as Death's Head II, to break free from Necker's control and continue on his adventures, where he met Tuck.
Eventually the two suddenly materialised on Maruthea, where they came across the Death's Head Interactive Archive. Death's Head II was implanted into the archive, forced to re-experience his past adventures and interact with a virtual representation of the original Death's Head. Tuck was captured by Hob and forced to watch the archive. However, Death's Head II was able to return to his body from cyberspace, rescue Tuck, and stop Hob from killing the original Death's Head. When the original tried to kill his future self, the Doctor incapacitated him with a modified TCE. He revealed he was the one who took Death's Head to the "robot universe", and that he had transported Death's Head II and Tuck to Maruthea. As Death's Head II and Tuck left, he wiped the original's memory of the encounter with his future self and returned him to the party. (COMIC: The Incomplete Death's Head)
Behind the scenes
Death's Head was a character who served as a central pillar holding together the so-called Marvel UK universe. He was one of the first of its creations, appeared as a guest (in his first two incarnations) in most of its ranges and interacted (as an ally and/or antagonist) with multiple staple Marvel heroes and teams, including the Hulk, She-Hulk, (two versions of) Spider-Man, (two versions of) Iron Man, Thor, the Beast, Fantastic Four, X-Men, Avengers, Novas, Guardians of the Galaxy, Young Avengers, etc. Perhaps, uniquely, he fought against the universal forces of evil in two universes: Unicron in Transformers universe and Galactus in Marvel universe. Hence, it was in the very nature of the character to serve as a link between realities by constantly hopping from one to another.
Death's Head was originally created to be a part of the Transformers line of comics published under a license from Hasbro. He was then given his own series in Marvel UK branch of Marvel Comics. Part of that transition was by way of crossing over with Doctor Who, where the Seventh Doctor and Tissue Compression Eliminator were used as a plot device to downsize the character from Transformers to normal size. After that, sometimes Death's Head appeared in DWM, other times DWM characters (such as Josiah W. Dogbolter or Keepsake) appeared in his series. The practical upshot was that the Doctor was a part of the wider Marvel Multiverse (or at least Megaverse), at least at the time.
In most of these crossovers, references to the Marvel universe tended to be subtle. For instance, COMIC: Time Bomb! ends with Death's Head on top of a building which fans might recognise as the home of Fantastic Four, which was only made overt in the next, non-crossover issue COMIC: Clobberin' Time! of the Death's Head series.
However, the stories COMIC: Party Animals and The Incomplete Death's Head make the Doctor's role on the Marvel comics universe much more overt, additionally stating that it was the Doctor who sent Head to the Transformers universe in the first place.
Marvel reference books place stories like The Transformers, Doctor Who and Spider-Man in different universes, with Death's Head himself listed as a temporary resident of at least two Marvel universes, Earth-5555 (year 8162) and Earth-8410 (year 2020), whereas the Doctor's universe is occasionally implicitly referred to as Earth-5556. However, very few comics featuring Death's Head mentioned trans-universe travel explicitly, instead just showing Death's Head jumping through time with whatever time machine available. Thus, for the sake of this wiki, we presume all Death's Head comics we cover take place in the same universe, unless stated otherwise.
In the 1980s-1990s, Marvel was licensed to produce both Transformers and Doctor Who stories, but after the licenses fell to other publishers the divide between their universes became more pronounced. This is where The Incomplete Death's Head gets its name -- from the reality that no single omnibus is likely to be able to completely publish his arc. This is additionally why the series never uses the word "Transformer" and makes this part of Death's Head past corrupted in Hob's archive. Ironically, this is also why The Incomplete Death's Head is so obscure. Because of its use of Doctor Who characters and Marvel characters at once, no one really has the right to reprint it.
The character of Death's Head has had several major iterations. The original one — that is, the star of 1988 Death's Head series — is the one that has had the most interaction with the Doctor. The Doctor and the second version of the character then interact in The Incomplete Death's Head.
However, other versions of Death's Head have never been featured in Doctor Who stories; this includes Death's Head 3.0, seen in Marvel US comics published in 2006, as well as both Death's Head 4.0 and Death's Head V, seen in Marvel US comics published in 2019.
- Death's Head at the Appendix to the Handbook of the Marvel Universe
- Death's Head at Marvel Database
- Death's Head II at Marvel Database
- Death's Head at Transformers Wiki (written in first-person)
- Death's Head at Transformers FANDOM wiki