Valdemar was an ancient entity believed to have been the last of the Old Ones. Stories and myths of it spread across star systems, as well as races.
According to legend, Valdemar was a black mass of life created or released by the Great Old Ones when they penetrated the higher dimensions. This force then swept across the universe like a stain, almost destroying the Great Old Ones. These myths also stated that it was a force that needed a lifeform to contain it. The possession was known as Stoodlhoo by the Alpha Centaurans, Prah-Tah-Cah by the Xanir and The Getting Into by the Ogrons.
The survivors were said to have killed Valdemar somehow and buried it on the planet Ashkellia. Stories existed on numerous worlds, including Gallifrey, that the entity survived and awaited its resurrection. In fact, the riddle of Valdemar and the Great Old Ones is number six on the list of great ten mysteries of the Universe.
The truth of the matter was far different. Valdemar was actually the result of an experiment by the Great Old Ones to penetrate the higher dimensions, a scientific study that went horribly wrong. Ashkellia was actually a massive particle accelerator for which the Palace of the Old Ones served as the control centre. The legendary Tomb of Valdemar was actually a gateway to the higher dimensions. When one of the Old Ones entered the gateway to act as a vessel for the higher dimensions, they realised that the process was destroying the universe. Then it changed itself and held the higher dimensions back and it was this entity that was in essence Valdemar. (PROSE: Tomb of Valdemar)