Reaper (Doctor Who)

Reapers are large dragon-like creatures that appear in the universe of Doctor Who. These beings are linked to time itself, acting as a form of "antibody" to cleanse infections in the timestream. In the event of a temporal paradox, the Reapers will be drawn to it and will sterilise the wound in time by devouring everything in sight.

Biology
The Reapers were flying dragon-like beings with red eyes, two wings, two claws below their wings that could break through stone, mouths on their chests, tails that resembled a scythe and dark brown, tough scaly skin. They lived in the Time Vortex as well as the Morass.

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.

Reapers had the ability to deactivate TARDISes as to not allow for an escape from their traps. They could also break their way into TARDIS interiors to reach those inside.

According to the Ninth Doctor, there was nothing in the entire universe which could harm the Reapers. 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.