The Dream Lord

The Dream Lord was a dark mental version of the Doctor, who possessed 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. This pollen induced a collective dream state for the Doctor and his companions as it was warmed up.

Biography
After his creation, the Dream Lord trapped the Eleventh 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. In actuality, the Dream Lord had created both worlds, all the while they 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 was truly defeated 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, the Dream Lord may never be truly defeated, but only hiding within him. (DW: Amy's Choice)

Personality
The Dream Lord drew his personality and observations from the 'dark' dimensions of the Doctor's subconscious. This included 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. The Doctor said to the Dream Lord not long after they met, that he knew who the Dream Lord really was because only one person hated the Doctor as much as the Dream Lord did. 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)

Appearance
Physically, the Dream Lord appeared as a short, older man. When he faced the Eleventh Doctor and his companions, the Dream Lord's appearance was generally a parody of the Doctor himself, consisting of a tweed jacket, striped shirt and bow tie strikingly similar but subtly different to the Doctor's. (DW: Amy's Choice)

Behind the scenes

 * The Dream Lord was similar to some prominent villains seen throughout the orginal series, most notably the Valeyard, who was also a manifestation of the Doctor's own dark side. The Dream Lord also shared some similarities with the Celestial Toymaker, the Master of the Land, the Black Guardian (who claimed to be the "evil to the Doctor's good"), and the Master (in as much as he is the Doctor's nemesis and yet defeats himself surreptitiously helping the Doctor and his companions). He also bore some resemblance to the Seventh Doctor, who was considered among the darkest of the Doctor's incarnations, a cunning manipulator, and master of mindgames.
 * Some fans have noted the similarity between the Dream Lord and the Star Trek character Q. Both characters are capable of manipulating reality and creating fictional worlds; both have a penchant for torturing or otherwise playing with the principal characters; both have a tendency to disappear in one place and reappear in another; and both have a flair for the dramatic, even going so far as to change costume to suit a location (or to better facilitate a joke). The key difference between the two, of course, is that Q is capable of altering actual reality, whereas the Dream Lord is only capable of manipulating dreams.