The Dark (The Twilight Streets)

The Dark were a group of entities who were allied with Pwccm in the fight against the Light and Abaddon.

Powers and abilities
The Dark were halogen lifeforms, though they seemed to be made of the opposite of light. Though not native to temporal rifts, they could live in them. They were capable of taking over people's minds, allowing them to control their bodies. When possessed, people would have black eyes, though only when the Dark was uncovered. (PROSE: The Twilight Streets)

History
The Dark and Pwccm were enemies of the Light and Abaddon. They had fought throughout time and space until they reached the Cardiff Rift. They were trapped within until 1876, when the Rift opened in Tretarri, allowing the demons to fight. Abaddon pretended to be defeated, allowing Pwccm to take control of the Rift.

Jack Harkness killed Abaddon. This meant that the Light was forced to fight the Dark and Pwccm alone. The Dark began to gain power and Bilis Manger had to ask Torchwood Three for help. When he kidnapped Owen and Toshiko and placed the Light in them, the Dark possessed them. The Dark continued to grow in power, taking over more and more people. Manger was able to remove the Dark from Toshiko and Owen and place it in Idris Hopper. With the Dark thus distracted, Torchwood was able to use the Rift and trapped the Dark in a special box.

Alternate timeline
In an alternate future where Bilis Manger never asked for help, Owen Harper, Toshiko Sato and later Gwen Cooper were possessed by the Dark. The Dark made them capture Jack Harkness and use his immortality to control the Cardiff Rift, allowing Torchwood to become the most powerful empire on the planet. (PROSE: The Twilight Streets)