Reaper

Reapers were beings linked to Time itself, searching for temporal paradoxes.

Biology
The Reapers were flying beings with sleek heads, four long hooked arms, mouths on their chests, tails that resembled a scythe and dark brown, scaly skin. (DW: Father's Day) They lived in the Time Vortex, (DW: Father's Day) as well as the Morass. (DWM: The Crimson Hand)

The Reapers would search out disturbances in time, such as temporal paradoxes, leave the Time Vortex and "sterilise" the area by devouring everything, with older objects being the tastiest and sentient beings of more interest than inanimate objects. They engulfed creatures in a flash of light with their mouths, leaving no traces. According to the Ninth Doctor there was nothing in the entire universe which could harm them. Despite these powers, they were material creatures, at least in this world, and could be blocked. They could not pass through solid objects and older objects, such as a church, were more effective barriers. As damage to time in the form of time paradoxes accumulated, it allowed them entry. They could also be banished if the timeline was repaired, returning everything as it was before and removing everyone's memories of the Reapers. (DW: Father's Day)

History
The Reapers were known to the Time Lords, who had ways to stop them. After the Time Lords were destroyed in the Last Great Time War the Reapers could enter into the universe.

When the Ninth Doctor brought Rose Tyler to 1987, she saved her father Peter Tyler when he was supposed to die in a car crash. This allowed the Reapers to enter the universe, where they began feeding on humans. The Doctor was able to hide some of the survivors in an old church, planning to bring his TARDIS to them. Unfortunately, Rose touched a younger version of herself, allowing a Reaper to enter the church, eat the Doctor and disrupt the TARDIS. The Reapers were eventually repelled when Peter jumped in front of the car, which restored the timeline. (DW: Father's Day)

Behind the scenes

 * The name "Reaper" never actually appears on-screen. The script to Father's Day described the Reapers as appearing as multiple Grim Reapers who cut their victims down with their scythe. The idea of a more alien look for the Reapers came out of production team discussions.
 * A number of people, such as Bee, were said to be late for the wedding, implying they were eaten by the Reapers.

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