Valdemar
Valdemar was an ancient entity that was believed to have been the last of the Great Old Ones with stories as well as myths of it spreading 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 which almost destroyed the Great Old Ones. These myths also state that it was a force that required life to contain its essence. The survivors, however, managed to kill it and buried Valdemar on the planet Askhellia. Stories existed on numerous worlds which included Gallifrey that the entity survived and awaited its resurrection. Infact, 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 as Valdemar was actually the result of an experiment by the Great Old Ones to penetrate the higher planes. A scientific study that went horribly wrong. Ashkrellia 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 realized that the process was destroying the Universe. Thus, it changed itself and held the higher dimensions back and it was this entity that was in essence Valdemar. (PDA: Tomb of Valdemar)
Behind the scenes
Valdemar is the Scandanavian form of the Russian name Владимир (Vladímir). The closest connection that can be made to H. P. Lovecraft is the Edgar Allen Poe story "The Facts in the Case of M. Valdemar" which can be traced as the principal influence of Lovecraft's "Cool Air".