Wild Hunt

The Wild Hunt is a group of specters, led by the King of the Wild Hunt, which is considered to be an omen of misfortune and death. It is said to appear mainly, but not exclusively during the winter.

Spoiler warning: Significant plot details follow.

Mysterious disappearances and reappearances of people who seem to be suffering the effects of some sort of time dilation are also blamed on this group of spectres. These individuals seemingly return to their loved ones decades later than they left, but apparently not having aged at all. In reality, the "Wild Hunt" were not spectres at all, but an elven cavalry lead by Eredin Bréacc Glas and known as the Dearg Ruadhri, or "Red horsemen" in Elder Speech.

Significant plot details end here.

In The Witcher computer game

The King of the Wild Hunt appears at several points over the course of the game. This spectre seems to dog our hero's every step. He also seems to take pleasure in spewing twisted interpretations of past events.

Journal Bestiary Entry

"The Wild Hunt is a horde of specters that roams the sky during storms and is an omen of disaster. The appearance of the Wild Hunt foreshadows war and woe, much as a comet does. The spectral Wild Hunt sometimes appears in nightmares of the cursed or those touched by Destiny."

Details

Occurrence:	The Wild Hunt appears in the sky as a harbinger of war and other misfortunes; some believe it to be simply a magical phenomenon and not a horde of specters; elven sources refer to it as the Red Riders
Immunity:	No information is available about fighting the Wild Hunt
Susceptibility:	Magic, especially of the ritual kind, according to the Hermit in the fields outside Murky Waters. They are apparently not fond of mandrake either
Tactics:	Warriors of the Wild Hunt do not have to fight, they inspire such terror that people cower after a mere glimpse; there are known instances of abductions by the Wild Hunt
Alchemy:	Vapors of the Hunt

Location

Source

Specters, Wraiths, and the Damned

Notes

So much for the lore, Geralt is not afraid of the King of the Wild Hunt. Signs and a silver sword both are effective.
During the Prologue, Lambert says in an optional chat that the Wild Hunt was around when Eskel and Vesemir found Geralt. He also mentions that Triss Merigold urged the witchers to go and drive the specters away.
During the Dead Hand of the Past quest, you will meet the King of the Wild Hunt, thus gaining a character entry on him.
The Hermit gives the quest Hunting the Wild Hunt. He describes the Wild Hunt as "...a group of crazed specters who traverse the heavens searching for souls like themselves. They are susceptible to magic, especially of the ritual kind."
In the Epilogue, the Geralt can have another conversation about the Wild Hunt with the Hermit. The conversation does not result in any new journal entries (not even a bestiary entry if you were missing it.)
If Geralt kills the King, he can loot the remains for Vapors of the Hunt with or without the associated bestiary and ingredient journal entries.

Quote from Sapkowski's books

Restless night, there's something bad in the air... Animals're beating in the barn... And in this wind screams can be heard...'

— Wild Hunt — witcher said quietly. — Close the shutters well, Mr. Hofmeier.

— Wild Hunt? — Bernie took fright. — Specters?

— Don't worry. It'll move high. It always move high at summer. But children can be woke up, The Hunt brings the nightmares. It'll be better to close the shutters.

— Wild Hunt — said Dandelion, looking up uneasily — heralds a war.

— Nonsense. Superstition. — Andrzej Sapkowski, Times of Contempt

cs:Divoký Hon lt:Wild Hunt hu:Vadak ru:Дикая Охота