Psychic pollen

Psychic Pollen is an entity that feeds on a person's dark side. The entity then turns a person's inner dark side against them and then allows that darkness to attack that person.

Technology
The Dream Lord could create various types of dreams and manipulate them, including an army of pensioners and a sun burning ice. This pollen was disposed of through the TARDIS doors into space (DW: Amy's Choice).

History
Some psychic pollen got into the TARDIS and put everyone into a dream state where the Dream Lord, a form of The Doctor, appeared to control. The Doctor eventually realised that both realities are 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.