The Dream Lord

The Dream Lord was a dark version of the Doctor, who apparently had control over dreams. He was "awoken" by specks of Psychic Pollen from the Candle Meadows of Karass Don Slava that had got stuck in the time rotor of the Doctor's TARDIS console, and induced a collective dream state for the Doctor and his companions as they were warmed up.

Biography
After his creation, the Dream Lord trapped the Doctor, Amy Pond and Rory Williams between two worlds, one in a seemingly idyllic Upper Leadworth five years in Amy and Rory's personal futures and the other in a TARDIS hurtling toward a "cold star". He told them that one reality was dream and the other was real, and that they would have to choose between them. Both involved deadly danger. Dying in the dream world would cause them to wake up in reality. Actually, the Dream Lord had created both worlds, all the while they had remained asleep in the Doctor's TARDIS.

The Dream Lord would appear to the Doctor and his companions in both worlds, taunting the Doctor on his insecurities and defects and pressuring Amy to assess her own priorities and 'dreams' for the future - whether she wanted a life of adventure with the Doctor or to be settled safely with Rory and a family. He seemed to relish attempting to bait the Doctor and his companions with remarks about their relationships to one another. The Doctor eventually realised that neither world was real, and managed to return them all to the waking world by destroying the TARDIS and seemingly killing himself after they all died in the Upper Leadworth world.

Whether or not The Dream Lord had been truly defeated at the end of his debut is open for interpretation. After he had found and removed the psychic pollen, the Doctor saw the Dream Lord's face in his reflection in the TARDIS console, smiling slyly at him. Being a representation of the Doctor's inner darkness, he may never be truly defeated, but only hiding in him. (DW: Amy's Choice)

Psychological Profile
The Dream Lord drew his personality and observations from the 'dark' dimensions of the Doctor's subconscious. This inculded his innermost thoughts and doubts about the course of his life and perhaps 'cavalier' treatment of others, including self-loathing, guilt, arrogance, selfishness and even lust. An additional layer to the Dream Lord's identity emerged when the Doctor described the Psychic Pollen which created the Dream Lord as a 'mind parasite'. (DW: Amy's Choice)