Psychic pollen

Psychic Pollen is a parasitical life form that feeds on the darkness in a person's mind.

History
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. The Dream Lord forced them to choose between two realities, telling them that dying in the dream would cause them to wake up in reality. The Doctor eventually realised that both realities were dreams and destroyed the TARDIS, killing himself, Amy and Rory, 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)