Sluagh

Slaugh are ghostly, supernatural beings (often considered fay) found in the folk tales of Celtic nations such as Scotland and Ireland, they are part of a larger group of entities known collectively as the Unseelie Court, as well as part of the older tradition of the Wild Hunt.

History
A Sluagh is a particularly feared and evil kind of undead fairy from Celtic folklore that exists as a collection of wicked spirits that fly across the land looking to capture mortal souls to add to their ranks, having been eternally exiled from any true afterlife - they tend to appear as flocks of crows and cause great destruction and ruin, though there are methods to defeat them, such as closing windows facing West (which is said to be where the storm of spirits would often come from).