Scourge

The Scourge were demonic eight-dimensional parasites which preyed on the depression, doubt and despair of their victims. When manifested in "normal" space they resembled large insects.

Biology
When physical, the Scourge resembled gigantic black mantis-like insects. They acquired this form by mutating human bodies. Upon invading a host body and taking over, the "shadow" of the Scourge would appear on the hand of the host in the shape of a burned pentagram. This would allow the Scourge to invade the host's mind. The Scourge were physically stronger than humans. The Scourge were also a hive mind, meaning that if one of them was trapped between dimensions, they all were. (AUDIO: The Shadow of the Scourge)

Powers
The Scourge's voice inspired religious awe in humans, causing them to feel guilt and obey their commands. They had complete control over the human nervous system, allowing them to force humans into autoasphyxiation, pushing past the genetic failsafe against self-destruction. When at full power, they could traverse all of time and space instantaneously and bend time and space to connect points. In order for the Scourge to invade a universe, they would need a sacrifice and would also need huge amounts of fear and despair at the moment they materialised. (AUDIO: The Shadow of the Scourge)

Being a creature who could only survive as part of a hive mind, should they ever find themselves alone, the Scourge would often infect the people around them by entering their minds and implanting false memories. This was once noted by River Song, who was made to believe that she was married to a host of the Scourge named Jane, after numerous related memories were implanted there. These memories were so strong that she was prepared to kill her own sister. A lone Scourge could also survive an unbelievably long amount of time alone, and even in the Void, the dead space between realities. (AUDIO: The Wife of River Song)

Technology
While they seemed capable of awesome powers without technology, the Scourge made use of a time travel experiments to enter N-Space. They also utilised bio-dimensional implants, which they used on their humans to stretch them across dimensions and prolong their mutation for centuries. (AUDIO: The Shadow of the Scourge)

History
The Scourge were said to be a part of humanity ever since the first mammal became aware of itself and felt guilty for it. They lived in another plane of existence which had geometry reminiscent of Hell, giving the universe its name. They were also said to exist within all humans as their despair.

The Scourge believed the multiverse was organised so that humans would be their food. They had always been in contact with the humans (specifically psychics) who felt despair, depression, and fear on the astral plane. They referred to themselves as the manifestation of fear.

Finding the right point where they could invade the universe, they discovered the Seventh Doctor while he dreamed on the astral plane and he offered them the chance to invade Earth and the cosmos. They met him in 2003 at the Pinehill Crest Hotel and took the building into a fractional dimension, halfway between N-Space and Hell. The Scourge Leader met with the Doctor who, on behalf of Gallifrey, welcomed the Scourge and made up Earth's surrender papers for the Scourge to see. Focusing, the Leader, Demi-Leader, and Bridgehead tried to bring the others to invade, but the Doctor, going back and forth, became one with them when they betrayed him. He helped the Bridgehead and Demi-Leader to de-mutate, but was forced to spread the Leader across the dimensions for millennia. (AUDIO: The Shadow of the Scourge)

During an expedition to uncover the Hive, after an attempt to rescue an abseiling archaeologist from the Void awent awry, River Song fell into a temporal rift and into an abandoned meditation facility. There, she discovered that all of the patients had seemingly been overcome by a virus that had driven them mad. The robotic staff had then murdered them as an act of pity. Together, with the help of her sister Brooke, and a version of herself of a fractured timeline, she discovered that the archaeologist Jane was the host of a long surviving member of the Scourge, that had used River to enter the Hive facility into the past, using River's fractured timeline, and infect the other patients, thus creating her new hive mind and enabling her to feed on them.

Jane had used her abilities to implant a fake collection of memories into the older River of them being married. When the present River arrived on the scene, the timelines began to converge, making their memories syncronise. Using this newfound trust, she attempted to fool River into believing her sister was the true source of the plague. But when all was revealed, the two Rivers and Brooke attempted to kill Jane by refusing to be scared of her, thus cutting off her food source. However, having been infected by the Scourge and escaping the Rift, Brooke passed the infection onto Hillier, the expedition leader, who had refused to quarantine the planet for fiancial reasons. She then threw him into the Void, having successfully passed the virus on, ridding herself of it, and isolating it in the Void. (AUDIO: The Wife of River Song)