Wild Hunt

The Wild Hunt was a haunting phenomenon, what some described as being a physical manifestation of good life force,  of wild frenzied hunting in which a being known as Cerunnos or the Master of the Hunt swept through the land with his hounds   and numerous other creatures.

Locations
The Wild Hunt was a phenomenon that occurred on many different planes of existence. It particularly occurred in the divine realm of Tir na Og, in the Feywild, in the Shadowfell, and on the Prime Material plane. On that plane of existence it's been sighted on Nerath, Toril, and worlds where the Celtic pantheon was worshiped. On the latter worlds, druids would build standing stones, believing that the Wild Hunt could use them as beacons.

Some believed that the Wild Hunt only occurred at certain times of year, such as during the autumn and winter. Some would say that the hunt of a quarry could go on days or weeks, while others would say a Wild Hunt only occurred during the night. Those who believed the former would sometimes claim that only a single Wild Hunt could occur on the Prime Material on any given night.

Manifestation Reason
The reason for the Wild Hunt occurring somewhere varied widely.

On worlds where the Celtic pantheon was worshiped, the Wild hunt would manifest in areas whenever there was a great force of evil. This could be a singular grossly evil act, an evil temple, an evil army, an evil priest or wizard, or an evil adventurer or demigod that had manifested evil among the Celts. The Wild Hunt would always manifest from the source of evil, which it then would seek out and attempt to destroy. If the Master of the Hunt and his hounds were slain, their bodies would disappear reconstitute themselves. They then would return each following night until the source of evil that had summoned them was vanquished.

In the Celtic pantheon's divine realm of Tir na Og, the Wild Hunt would manifest itself to put an end to anyone mucking about with the freedom of the realm.

History
Around the mid-14 century DR of Toril, the Wild Hunt was known to appear in the area around the city of Phlan in the Moonsea lands of north Faerûn. It was only a rare occurrence and cause of the phenomenon was unknown to the locals, yet around that time folks were warned to stay indoors in order to avoid being caught up in it.