Old One (Tomb of Valdemar)

The Old Ones were beings that existed when the universe was young, and then mysteriously disappeared.

History
The First Doctor and the Master, whilst still at the Time Lord Academy, broke into the Matrix to learn about the Old Ones. It related to them that the race existed further back in history than the range of a TARDIS. They released an entity known as Valdemar into our universe. They fought and defeated it, but could not destroy it, having to bury the thing. What happened to them after that was unknown. The mystery of Valdemar and the Old Ones became the sixth of the great mysteries of the universe.

The truth, though, was not so mythic. The Old Ones, whose great psychic powers made them more adapted to perceive the higher dimensions than most inhabitants of our universe, wanted to explore those dimensions. They had gone to other planets, capturing their inhabitants and dissecting their brains to locate organs that allowed those species to perceive the higher dimensions. On Earth, they experimented on humanity's ape ancestors, finding a vestigial receptor inside the hypothalamus. This helped them develop a neural-inhibition vaccine that reduced the effects of the higher dimensions on them.

Then they built Ashkellia, a planet-sized particle accelerator that would open a breach to those dimensions. They succeeded, but the results were catastrophic, destroying their race. Valdemar sacrificed itself to seal the breach and slumber, acting as a barrier between the dimensions.

The Fourth Doctor and Romana discovered this when they stumbled on Ashkellia whilst searching for the Key to Time over a million years after Valdemar's self-imprisonment. (PROSE: Tomb of Valdemar)

Biology
The Old Ones were immense, the size of buildings. They were green with veiny purple stalks growing out of one end, a bluish globe-like head and tentacles. They had great psychic powers, including telepathy. (PROSE: Tomb of Valdemar)

Technology
The Old Ones had developed transmat technology, a neural-inhibition vaccine that reduced the effects of the higher dimensions and telepathically responsive architecture. The Palace of the Old Ones looked like it was built for humans because when it was discovered by humans, it remodelled itself for them. It also was self-maintaining and self-regenerating, reminding the Fourth Doctor of the city of the Exxilons. (PROSE: Tomb of Valdemar, PROSE: Death to the Daleks)

Symbology
The sign of the Old Ones was an evil-looking five-pointed star. (PROSE: Tomb of Valdemar)