The Entity (From Eternity)

The Entity was a creature which perceived Time backwards. It was interrogated by an unknown incarnation of the Doctor on behalf of the Time Lords after it had caused countless deaths.

Biography
The Entity believed itself to have been the first intelligence born into the Universe, spending aeons of darkness alone until it saw the first speck of physical matter come into existence. As further aeons passed, it saw the first stars blossom. Eventually it made contact with an intelligent species, only to see the species devolve into savagery, gradually becoming less sentient until all that was left was primordial ooze. When the same happened to the next minds that the entity encountered, it concluded that this bizarre infection was built into the nature of the Universe. Fearing it would be alone for all of eternity, it tried to warn species of the danger they faced. When they failed to respond to the warning, the desperate entity tried to force them to do so by presenting them more immediate dangers. The first species it destroyed was the Iarcho, and many more followed until it finally learned the truth from the Time Lords about its perception of time. It had been witnessing evolution and progress in reverse. Unable to correct its past (or rather future) mistakes, the entity accepted that it has performed monstrous acts for which it could never atone. It accepted that it would spend the rest of its long life alone until it died at the beginning of the Universe. When this finally occured, the entity discovered that the Universe was just one of many, separated by dimensional membranes that occasionally collided and interacted, creating new Universes and new minds whenever they touched. The entity was at peace, knowing that its long periods of loneliness were merely a phase that would be interspersed with moments of brief and intimate contact with other minds. (ST: From Eternity)