The Entity (TARDIS)

The Entity was an intelligent, malevolent, golden ball of energy that was imprisoned by the Doctor in a special vase for murdering others by eating the time out of their lives.

Biology
The Entity fed on the Time energy of living beings, until they died of old age. The process took about an hour on the average human. Whilst the Entity was feeding, the individual it was feeding on appeared to have a golden "shell". On its own, the Entity appeared as a golden ball of energy floating in mid-air. (VG: TARDIS)

History
At some point in his past, the Doctor met the Entity, and placed it in a vase inside a safe in the TARDIS drawing room.

When the Eleventh Doctor was sucked into a spacetime riptide, Amy looked in his drawing room for a tractor beam device to pull him back inside the TARDIS. Unfortunately, she knocked down the vase holding the Entity and it escaped. Later, when the Doctor was back in the TARDIS, a lesion in time caused Amy to be pulled into the TARDIS one thousand years in the future, where the Entity started feeding on her. The Doctor built a Tachyon Feedback Loop, which he sent to Amy to pull her back to the present. He told the Entity to stop feeding on her. The Entity agreed and tried to feed on the Doctor. However, the Doctor trapped the Entity in another vase and told it he would release it into the riptide with the Chronomites, which the Entity could feed on for eternity. The Doctor released the Entity to the Chronomites, but forgot to mention they were very itchy. (VG: TARDIS)