Time Assassins

Time Assassins are a race of teleporting Humanoid species, who can absorb the time people have left to live before they die. They use that 'time energy' to give themselves power. In their standard states they are disfigured creatures, but once they absorb enough energy they will turn into a more Human form, and eventually look like a real Human.

Time Energy can also be obtained by absorbing time travellers. Because they have travelled through time, time travellers contain lots of time energy.

The Time Assassins also have a leader. 25% of every amount of time energy they absorb from a person goes to the leader. The leader is the strongest and biggest of them all, and requires as much time energy as possible. Once the leader is in full form is can destroy entire cities at once.

A long time ago the Time Assassins were locked away in a Pocket Universe by the Time Lords, but when the void between the worlds was crumbling apart because of the Daleks creating their Reality Bomb (as seen in The Stolen Earth and Journey's End), they managed to escape and return to the universe.