The Dark (Forever Dreaming)

The Dark was a consciousness that the Eleventh Doctor and Amy encountered while trapped in the dream world of Psychspace.

Biology
The Dark appeared as tall men in suits and bowler hats while in Psychspace. They were constantly grinning, and were sometimes shown as having jagged teeth. They each had an individual colour (such as red, green, or blue), and this corresponded to everything from the colour of the rosettes that they wore to the colour of the individual design on their glasses. When the Dreamers let the Dark absorb them, the dreamers took on an appearance similar to that of the Dark, becoming all shadow and gaining one of the designs on their glasses.

History
The Dark presumably appeared when the Psychic Squid began trapping great thinkers in Psychspace. They attacked the dreamers and forced them to enter a coma-like state just to keep the dream world from unravelling. When the Doctor and Amy arrived, they attacked Amy and then the Dreamers. The Dreamers let themselves be absorbed by the Dark, and then attacked the Psychic Squid, destroying Psychspace, and presumably the Dark, in the process.