The Wild Hunt is a group of spectres in the Witcher series. Led by the King of the Wild Hunt, this spectral horde is considered to be an omen of misfortune and death. It is believed the Wild Hunt abducts people. People who believe themselves to be kidnapped by the Wild Hunt may return, although they will be suffering the effects of a time dilation of some sorts. One such individual spent a year with the Hunt and when he returned, he saw the graves of his children.
The Wild Hunt aren't actually spectres. Instead, they are the Elven cavalry unit known as the Dearg Ruadhri ('Red horsemen' in Elder Speech) from Tir ná Lia, and their leader is Eredin Bréacc Glas. Tir ná Lia is actually not part of the human world, which is why they appear as spectres when they pass through. They do abduct humans, but time moves slower in the world of the Aen Elle that a year there could be an entire lifetime in the human world.