The Entity (TARDIS)
The Entity was a golden ball of energy who was imprisoned by the Doctor.
Biology
The Entity is a creature that feeds on time energy of living beings, until the creature dies of old age. The process takes about an hour on the average human. The Entity appears as a sort of golden ball of energy floating in mid-air.
It is unclear whether the Entity is a unique being or a part of a whole species of Entities.
History
At some point in his past, the Doctor met the Entity, and placed it inside a vase in a safe in the TARDIS drawing room.
When the Doctor was sucked into a time rift, Amy looked in the Doctor's drawing room for a tractor beam device to pull him back inside the TARDIS. Unfortunately, she knocked down a vase containing the Entity, and the Entity escaped. Later, when the Doctor was back in the TARDIS, the rift caused Amy to be pulled into the TARDIS one thousand years into the future, where the Entity started feeding on her. The Doctor used a device to pull Amy back to the present TARDIS, and the Doctor told the Entity to stop feeding on Amy. The Entity agreed, and tried to feed on the Doctor. However, the Doctor trapped the Entity in another vase, then tells the Entity that he will release it outside with the Chronomites, creatures that the Entity could feed on for eternity, since they have infinite time. The Doctor released the Entity to the Chronomites, but forgot to mention that the Chronomites are very itchy. (VG: TARDIS)
