Psychic pollen

Psychic Pollen was a parasitic life form from the Candle Meadows of Karass Don Slava. It sent people to sleep and fed on the darkness of their minds.

Some psychic pollen got into the TARDIS' time rotor, "awakening" when it was warmed up and placing the Doctor, Amy Pond and Rory Williams into a dream state. Latching onto the Doctor's mind, it created the Dream Lord, a manifestation of the dark side of his subconscious. It didn't latch onto Amy's or Rory's minds because it would of starved as they don't have anywhere near as much darkness as the Doctor does. The Dream Lord forced them to choose between two "realities", telling them that dying in the dream would cause them to wake up in true reality. After all three had died in one of the realities, the Doctor eventually realised that both realities had been dreams. He destroyed the TARDIS in the second reality, killing himself, Amy and Rory, thus causing them to wake up in the real TARDIS. The pollen was then found and blown out of the TARDIS by The Doctor. (DW: Amy's Choice)