The Absolute was a being that tried to alter the perspective of history.
The Absolute was unable to understand the various versions of history that existed because of perspective. It made numerous attempts to possess humans so that it could see history from this point of view. It attempted this with Miquel Domínguez, who died in the process, then upon George Orwell with greater success, separating his consciousness from his body. The consciousness of those possessed held conflicting accounts of historical events and so the Absolute expelled them and they formed a separate creature. Unable to decipher a correct version of history, the Absolute was driven insane.
The Absolute attempted to remove all perspective from history so that it could just be one string of events. This allowed time travellers to see different perspectives upon events all at once.
The Eighth Doctor was one of the few able to see the Absolute. He chased it for some time and managed to collide it was the consciousness entity. This restored history to its earlier state. (PROSE: History 101)