Black Guardian

"Destroy the light and you destroy yourself. Darkness can not exist without the knowledge of light."

- The White Guardian """Nor light without dark."

- The Black Guardian

The Black Guardian, played by Valentine Dyall, is a villain from the Doctor Who series. He is a personification of chaos and opposes the White Guardian. The Black Guardian seeks for the destruction of the universe and of the White Guardian.

Quest for the Key of Time
When the White Guardian sent the Doctor and Romana to find the scattered segments of the Key of Time, the Black Guardian looked for them as well. After his tries to prevent the two Time Lords from assembling the Key failed, he posed as the White Guardian and asked for the Key, but the Doctor looked trough his disguise and scattered the Key again.

Enlightenment
The Black and the White Guardian let the Eternals start a race against each other and promised Enlightenment to the winner. The Black Guardian assisted a female Eternal named Wrack to sabotage the other competitors by giving her a bit of his power and also used the Doctor's companion Turlough, who has been saved by the Guardian, as his mole to destroy the Doctor. His plan was to have the Eternals spread chaos across the universe once they had Enlightenment. However the Doctor threw Wrack overboard so he and Turlough were the ones who approached the two Guardians. The Black Guardian offered Turlough Enlightenment in return for sacrificing the Doctor, but Turlough refused and threw the crystal at the Guardian who vanished in flames. The Doctor then stated that Enlightenment has never been the crystal, it was the choice. The White Guardian agreed, but warned them that the Black Guardian is still alive and will exist until the Guardians aren't needed anymore.

Trivia

 * He his very similar to the Trickster: they're both manifestions of chaos, both seek for the end of the universe and both saved people from death