The Trickster was an extra-dimensional deity with great power over Time. A member of the Pantheon of Discord, he often faced the Doctor or Sarah Jane Smith, either as hisemf (TV: The Wedding of Sarah Jane Smith) or through his own agents, known as the Trickster's Brigade. The Trickster generally sought to create chaos and misfortune by tricking mortals from the Universe into making deals with him to alter Time. (TV: Turn Left, COMIC: Turn Left Prequel Comic, etc.)
He notably appeared to a young woman who had once been wealthy, but was reduced to working as a two-bit fortune teller working on the crowded market-streets of Shan Shen in the 51st century, at which she did not have much success. When her despair had grown great, the Trickster appeared to her in a mirror, offering her renewed prosperity if she cared for the Time Beetle for him and did exactly as he said, (COMIC: Turn Left Prequel Comic) meaning she was asked to find victims whose timelines the Beetle could feed on by forcing them to relive and alter important choices in their lives. The Beetle was ultimately killed when Donna Noble managed to reverse the alteration to her timeline from within the altered world, whereupon the Doctor recognised the Beetle as a member of the Trickster's Brigade. (TV: Turn Left)