Garvond

The Garvond was a monstrous psychic entity or a gestalt born of the evil and demented thoughts of Time Lords when their minds were imprinted into the Matrix. Early on his life, The Doctor realized the Garvond had to be the collective evil of every single Time Lord; therefore, by erasing his own mental imprint from the Matrix, he was able to deny the Garvond existence. However, Mortimus, the Meddling Monk, used the powers of Artemis, a Chronovore, to erase that act of the Doctor's, restoring the Garvond and releasing it into the Universe to seek out the Doctor as a part of his plan to dominate all existence.

However, the Garvond's power was not infinite. The Doctor provoked it into attacking and shunted it into a copy of The Worshipful and Ancient Law of Gallifrey. Given the book's dimensionally transcendental properties, the Garvond was imprisoned within its pages, allowing the Doctor to dispose of the book in a dimensional pocket and destroy the threat of the Garvond.

The Garvond's powers were mostly linked to its Time Lord origin, allowing it to manipulate time in a localized area to great effect, aging and deaging things and living beings. When confronted by the Doctor, the Garvond was heavily damaged and sought to regenerate by changing history and allowing Time to try to repair the damage, feeding off the resulting energy; it could also feed off the power of paradoxes. It could seize control of artificial devices and inflict massive psychic damage to large groups of people. (Virgin New Adventures, The Dimension Riders)