The Dream Lord is the dark side of the Doctor, who has control over dreams. He was "awoken" by speck of psychic pollen from the candle meadows of Karass Don Slava that had gotten stuck in the TARDIS, and woke as it was warmed up. The Dream Lord drew his personality from the Doctor's inner darkness.
He trapped the Doctor, Amy Pond and Rory Williams between two worlds, telling them that one was dream and one was real and that they would have to choose between them. While in the dream world, he would appear to the group and the Doctor individually, taunting him on his insecurities and defects. However, the Doctor eventually realised that neither world was real, and managed to return them all to the waking world. After removing the pollen, the Doctor saw the Dream Lord's face in his reflection in the TARDIS console, smiling slyly at him. (DW: Amy's Choice)
Notes
- The Dream Lord shares some similarities with a few prominent villains seen throughout the classic series, including the Valeyard (like the Dream Lord, a manifestation of the Doctor's own dark side), the Celestial Toymaker, the Black Guardian (who claimed to be "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).
- Whether or not he had been truly defeated at the end of his debut is open for interpretation, as he was seen smirking at the Doctor at the end in the latter's reflection. Being a representation of the Doctor's inner darkness, he may never be truly defeated, but only hiding in him. This notion is supported by the Dream Lord's magnanimous decision to "withdraw" after the Leadworth dream ended.