Somnifax

The Somnifax was an engineered creature which could bring one's nightmares to life. The Sixth Doctor, Constance Clarke, Flip Ramon and Calypso Jonze followed it to 1937 where it entered the mind of H. P. Lovecraft seeking to take the form of Cthulhu. After it did so, the Doctor was able to trap it. (AUDIO: The Lovecraft Invasion)