Not-thing

The not-things, also called nobodies, were entities from the Void beyond the universe that could imitate others visually from their thoughts and memories. 
While the ones met by the Fourteenth Doctor and Donna Noble when the TARDIS deposited them on a spaceship at the edge of creation were seen to speak to each other, (TV: ) the Doctor was unsure if there were actually two not-things or if it was a single entity taking on two forms. (PROSE: )
== History ==
Whilst drifting in "the lack-of-light" of "the nothing", the not-things became aware of the rest of the "noisy, boiling universe", becoming shaped by their inhabitants' lives of "blood, fury, and hate", and turning malicious as a result.
When a spaceship arrived at the edge of the universe from a wormhole, the not-things boarded, seeking to use the ship to enter the known universe and start their own war campaign. In a desperate attempt to stop them, the captain of the ship started an extremely slow self-destruct process, and took her own life before the not-things could assimilate the knowledge of what she had done.
Three years later, Donna Noble and the Fourteenth Doctor ended up on the spaceship when the TARDIS malfunctioned, and the Doctor's attempt to repair the TARDIS by putting it into self-regeneration activated the Hostile Action Displacement System, resulting in the TARDIS fleeing due to the presence of the not-things, who revealed themselves to the Doctor and Donna after they assimilated their forms. The not-things become more aware of the others' knowledge the longer they hunted them, almost managing to deceive the Doctor and Donna several times, but had been unable to keep up the façade for long. Ultimately, the Doctor was able to deduce the captain's scheme of setting the ship to self-destruct, but his conclusion only alerted the not-things to the plan. 
They quickly raced to stop the self-destruct, but were not quick enough and the ship began to destroy itself when the Doctor sped up the process. With the danger having essentially passed, the TARDIS returned, but the Doctor was tricked into taking a not-thing with him instead of Donna, just as the self-destruct activated and killed the not-thing impersonating the Doctor. However, before the blast could reach Donna, the Doctor, having discovered he'd been tricked, returned and rescued Donna, throwing the not-thing from the TARDIS doors and leaving it to die in the blast. (TV: )
== Powers and abilities==
The not-things were capable of replicating individuals almost completely, including their memories. The Fourteenth Doctor claimed they accomplished this by converting the surrounding heat into mass, making the surrounding air cold when within their presence. However, as they acclimatised, they could do so without noticeable heat change. They copied best when the minds of their targets were at their most active, working off of fear and the adrenaline response for this effect, or even simple curiosity. As the not-thing copying the Doctor stated, when the Doctor had an idea, it meant he did too and became as clever as he was. 
The not-things were capable of assimilating thoughts and memories and even had access to memories and concepts that their targets weren't actively aware of. The not-thing copying Donna Noble had knowledge of the events of the Flux that Donna also had, but Donna wasn't able to see the memories clearly, likening it to "looking into a furnace". 
However, the not-things had trouble with shaping humanoid arms and jaws specifically, as well as the number of certain body parts, like the number of total knees on a human's leg. Additionally, being entities from outside the universe, they initially did not understand size, shape, object permanence, or the intricacies of the human psyche, which made them easier to identify, but this became more difficult as they learnt more about their target. By the end of their encounter with the Doctor and Donna, one was able to copy Donna to 99.9 percent, but her wrist was 0.06 millimetres off which the TARDIS could detect. They could take full advantage of their unique physiology, such as moving far faster than normal when running in a quadrupedal form. (TV: )
Category:Shapeshifters
Category:Non-existence
Category:Individuals who have been inside the Doctor's TARDIS
Category:Look alikes of the Doctor