The Sluagh are part of Celtic mythology. Lost evils souls who take the form of birds, they fly over the countryside, forbidden from ever touching earth. They appear to fly together, as a flock of black birds. Their souls were rejected by heaven, hell, and the earth itself because they were so evil. They are constantly searching for dead or dying individuals so they may kidnap their souls as well.
Seeing the Sluagh is considered a bad omen. It’s believed that the Sluagh always enter homes through west-facing windows so people would often close the west windows of their home in fear of these creatures.
Many legends and stories associate black birds as omens of evil or misfortune. Perhaps there is just something creepy-looking about them. It’s easy to see how people may have seen large flocks of eerie-looking birds and thought there was more to them then met the eye.