Reapers were beings linked to Time itself. They searched 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. (TV: Father's Day) They lived in the Time Vortex, (TV: Father's Day) as well as the Morass. (COMIC: 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. Older objects were stronger against them and sentient beings were of more interest than inanimate objects. They devoured creatures through their mouths, leaving no trace. They also released a golden glow from their eyes and under their skin when attacking, or about to attack.
According to the Ninth Doctor, there was nothing in the entire universe which could harm them. Despite these powers, the Reapers were material creatures, at least in this world, and could be blocked. They could not pass through solid objects. 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. If this happened, everything would return to as it was before and memories of the Reapers would disappear. (TV: 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, there was virtually nothing to stop the Reapers from entering the universe when a wound in time appeared.
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. They began feeding on humans. The Doctor hid 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, devour the Doctor and disrupt the TARDIS. The Reapers were expelled from the universe again when Peter restored the timeline by jumping in front of the car which should have killed him. (TV: 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.
- For unknown reasons they haven't appeared at disturbances in other fixed points, primarily during the Eleventh Doctor's era.
- Though all trace of them was supposedly erased, the Reaper's intrusion of Earth was in the Atraxi's records. (TV: The Eleventh Hour)