The Dark was an evil, sentient force born from the remains of the void before the Big Bang and the creation of the universe.

The Dark was shredded by the Big Bang, its physical being burnt, but its mental essence lived on and spread across the universe like a shadow until it coalesced among the primal matter of the planet Akoshemon. It began to influence the natives, who became embroiled in war and conflict. It finally achieved a single, corporeal form but was killed in 1482 by a group of humanoid travellers who learned of its existence.

Yet a creature such as the Dark was not bound by the normal rules of physics. Its mind survived in the ashes of its body. It planned to return by influencing the creation of the Bloodhunter which would provide it with a continual source of human blood, and thus managing to reconstitute itself physically. It influenced the Fifth Doctor's companion Nyssa and forced his TARDIS to appear on the world.

Even though the Dark was not bound by most of the normal laws of the universe, it had to obey some of them to exist. It could become corporeal in complete darkness (with tentacles of shadow and a cavernous maw), eventually taking on a crude form of the Fifth Doctor. It was now capable of psychokinesis. Yet it was not immortal as a result of its manifestation. This ended in it being destroyed and permanently killed by fire. (PROSE: Fear of the Dark)