Sluagh

The Sluagh are a type of spirit featured in Irish folklore.

Pre-Christianisation
Before the arrival of Christianity, Sluagh were a race of wraithlike spirits who formed a ghost hunt, riding in the night with a pack of spectral dogs. They raced against Death itself to snatch away the souls of the recently dead and reshape them into new additions to their dog packs.

Post-Christianisation
After the Christanisation of Ireland, the Sluagh were now said to be the souls of men so evil that even Hell could not hold them. They flew by large birdlike flocks, coming from the West, and sought out the dying to snatch their soul and consume it. Sluagh would occasionally attempt to steal a living man's soul, which is extremely rare; in such a case, the only way to escape this grisly fate would be to sacrifice another in your place, though this act would doom you to become a Sluagh yourself upon death.