Discordia
The Discordia were a race that resembled devils. They took the form of the greatest fears of their prey. With their transformation abilities, they could morph their limbs into armaments and tools of various stages of technological advancement. They could easily move throughout time, often using foreknowledge in order to retro-engineer their technology and avoid unwanted outcomes, or to reverse them afterwards.
History
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After the Last Great Time War, the Discordia discovered time travel technology on their home planet and began to spread chaos throughout time, with no one left to oppose them. (AUDIO: Time in a Bottle)
They were placed in a dimensional prison. One Discordia pretended to be Keira Sangstrom, an old Time Agent acquaintance of the Twelfth Doctor, and began travelling with him.