Dark

From TARDIS Index File, the free Doctor Who reference.

For the Darkness and the Dark see separate articles.

The Dark was a sentient evil force that is the remains of the void prior to the Big Bang and the creation of the Universe.

It was shredded in the process and spread across the universe like a shadow until it managed to coalesce among the primal matter of the planet Akoshemon. Once there, it began to influence the natives who became embroiled in war and conflict. It finally achieved the ability to make a single corporeal form for itself but was killed in 1482 AD by a group of humanoid travelers that learned of its existence.

It was killed through the use of fire and the body was placed in a crypt on the planet's moon where it's threat seemed to have ended. However, a creature such as the Dark was not bound by the normal rules of physics and it's mind survived in the ashes of its body. Thus, it planned to return by influencing the creation of the Bloodhunter to provide it with Human blood so it could reconstitute itself. It even influenced Nyssa forcing the Doctor's TARDIS to appear on the world.

While the creature was above the normal laws of the universe, it had to obey some of them in order to exist within it. One such factor was it could become corporeal in complete darkness and it accomplished this where it took a crude form of the Doctor. The Dark was now capable of psychokinesis as well but was not mortal as a result of its manifestation. This resulted in it being destroyed and permanently killed by fire. (PDA: Fear of the Dark)