Reapers
The Reapers were first seen in Father’s Day (Series 1 : Episode 8) against the Ninth Doctor as creatures that feed on ‘wounds in time’. They are flying creatures similar to pterosaurs, with multiple limbs, large wingspans and sharp teeth both in their beak-like mouths and their torsos. The acuminated to paradoxes in time in a similar fashion to the way bacteria acuminates to a wound. The Reapers attempt to sterilize the wound in time by devouring everyone in sight. They did this by engulfing their victims in a flash of light and leaving no trace of the person behind. It was possible to defend yourself from a Reaper with barriers, but this did not keep them out forever. The older the barrier, such as a church (as used by Rose Tyler, Pete Tyler and the wedding party), the stronger the defence, but if paradoxes occur within the safe haven, then the Reapers can enter.
The Reapers were powerful enough to devour most of the human race within a few hours in the alternate timeline after Pete was saved by Rose. The Doctor attempted to defeat them by drawing his TARDIS to him, but Rose touched her younger self, creating a larger paradox and allowing them into the church, where they devour the Doctor. It was said that the Time Lords used to stop Reapers from appearing by being able to fix paradoxes quickly, but when the Time Lords were wiped out, the Reapers were able to materialise anywhere there was a paradox as there was no other agency that could control them.
The Reapers could also be beaten by restoring the previous timeline, thus banishing them back to the Time Vortex. This is how Pete got rid of the Reapers in Father’s Day, as he threw himself in front of the car that should have killed him, restoring the original timeline. When such an event occurs, everyone who was devoured is brought back and put back the way it was, leaving no trace that they were there physically, as well as humans being unable to remember them appearing.