The Sentience (Nightshade)

The Sentience was an ancient and powerful force that lived inside the Earth, focused under Crook Marsham.

Biology
The Sentience took the form of a mass of energy. It was able to send out columns of energy from its central mass under Crook Marsham, which could change shape in order for it to feed.

The Sentience fed on energy, including those of humans. It couldn't feed on humans directly and instead needed to latch on to strong memories, such as regrets and desires. The Sentience would then create individuals from the memories and try to interact with the target. The stronger the belief in the memories was, the more powerful the copy was and the easier the Sentience could feed. After coming into contact with the target, the Sentience would feed on all the energy of the human, leaving a desiccated corpse that would disintegrate with a touch.

The Sentience also developed other powers to help it feed. It could cut off all telephone lines around Crook Marsham, stopping the inhabitants from calling for help. It was able to possess Billy Coote, but the energy eventually destroyed him. It could set up a force field that would make anyone who tried to escape sick and fearful. As it grew in power, it could also leave Earth and travel through time and space at great speeds. (PROSE: Nightshade)

History
The Sentience was present during the creation of Earth and was trapped inside the planet as it formed. For billions of years, the Sentience was too weak to react much. The low population in the area meant that there was little for it to feed on. One of the few times it was active was in 1644, where it fed during the Civil War.

In 1968, a radio telescope was built on top of where the Sentience was. This increase in both population and energy awoke the Sentience, and it started to feed on the inhabitants of Crook Marsham. It was eventually confronted by the Seventh Doctor, who showed it the energies given off by Bellatrix going nova. The Sentience followed the fossil energies back through time to 1644, destroying the castle of Crook Marsham along the way. It then went to a supernova in the Andromeda Galaxy, but was trapped when it turned into a black hole. (PROSE: Nightshade)

Behind the scenes
In the reference work I, Who, Lars Pearson speculates that the discorporated Sou(ou)shi become the Sentience.