The Entity

Details regarding the Doctor's earliest encounter with the Entity are sparse but it's clear he was forced to imprison the creature as it was devouring its way through the universe. And so the Entity was trapped like a Genie in a bottle, until one day, Amy Pond broke the bottle and released it...

Fact titleFact data
Home Planet:
Variable. But in its natural gaseous state it appears, to quote the Doctor, 'a sort of glowy, blobby thing'...
The Entity can be trapped and held for periods of time.
Start to worry when:
You knock over what looks like a glass bottle and gaseous contents escape. You may just have destroyed the universe. Oops.
First Appearance:

The Entity feeds on time energy from the past and can therefore make its victim cease to exist. The Doctor's dilemma was simple: he was reluctant to keep the Entity imprisoned but could not risk it consuming innocent people. He therefore unleashed the Entity on four-dimensional void creatures known as the Chronomites. The Entity could feed on their past, only to see them circle back in time unharmed.

I must feed!
The Entity

Of course, the potential flaw in the Doctor's actions is that it leaves the Entity at liberty...