Not-thing

The Not-Things were entities from beyond the universe that imitated the Fourteenth Doctor and Donna Noble when the TARDIS deposited them on a spaceship at the edge of creation.

History
At some point in time, the Not-Things became aware of the rest of the "noisy, boiling universe", becoming shaped by the "blood, fury, and hate" of the entire universe, becoming malicious as a result.

The Not-Things boarded a spaceship that arrived at the edge of the universe. In a desperate attempt to stop them, the captain of the ship started an extremely slow self-destruct process, but committed suicide before the Not-Things could assimilate the knowledge of what the captain had done.

Three years after, Donna Noble and the Fourteenth Doctor ended up on the spaceship, three years after when the TARDIS malfunctioned. The Doctor attempted to repair the TARDIS by putting it into self-regeneration, which activated the Hostile Action Displacement System due to the presence of the Not-Things.

Two Not-Things appeared before Donna and the Doctor was they were separated, each disguising as the other. The Doctor and Donna quickly realised what was happening as the Not-Things' arms were too long.

After the Doctor and Donna were separated, the shapeshifters started to become more aware of the others' knowledge, almost managing to completely deceive the real Doctor and the real Donna, but had been unable to keep up the façade. After regrouping yet again, the Not-Thing Doctor revealed their intentions; to travel to the universe and participate in the chaos and war of the universe, and win.

However, the Doctor also realised that the Not-Things relied on the active thoughts of their targets, encouraging Donna to not think, which was advice that not even he could follow for long. In the end, he succumbed to curiosity, before ultimately deducing that the ship was set on a very slow self-destruct sequence by the captain in such a way that the Not-Things weren't able to find out.

The Doctor was able to hold them off for a short while, but they raced to stop the self-destruct. The TARDIS knew that they wouldn't be able to stop it, resulting in the shutdown of the HADS and the TARDIS materializing. Donna and her Not-Thing both attempted to convince the Doctor that they were the real Donna, wherein the Doctor had chosen incorrectly, only noticing after being warned by the TARDIS. The false Donna was ejected from the TARDIS doors, killing her in the blast.

Powers and abilities
Being capable of replicating individuals almost completely, including their memories, the Not-Things were capable of copying Donna up to a 99.9% accuracy level. The 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. They had trouble with shaping human arms and jaws specifically.

Being entities from outside the universe, they initially did not understand size, shape, object permanence, or the intricacies of the human psyche. This made them easier to identify, however this became more difficult as they learnt more about their target as they continued to replicate them. 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 millimeters 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. However, their shapeshifting abilities have been shown to be chaotic to such an extent that two Not-Things suddenly grew to gigantic proportions, getting themselves wedged in a large corridor.

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 of Donna Noble had knowledge of the events of The Flux which the regular Donna also had, but Donna wasn't able to see those memories clearly, likening it to "looking into a furnace". (TV: )