The Mi'en Kalarash, or "blue fire", was an ancient myth from Gallifrey, said to inhabit "the wasteland between realities".
It fed on nightmares. The creature itself was composed of a series of minute slivers in incremental time, a "time magnet". Its outward appearance was a tall, thin humanoid figure with dark leathery skin, a bestial face, all swathed in blue fire. Its blue fire stimulated the victim's limbic system, feeding off the resultant emotion. (AUDIO: House of Blue Fire)
The Seventh Doctor thought the Mi'en Kalarash picked up his temporal trace and was tracking him, seeking to manipulate and corrupt impressionable military minds through the Blue Fire Project to bring about the end of the world. Already the Mi'en Kalarash had been present on Earth, manipulating Nurse Eve Pritchard for twenty years after it occupied her mind while she was under sedation following a car crash when she was four years old. (AUDIO: House of Blue Fire)