Smoughnigh

The ancient ghosts of Aernigh myth. While many of the Deinain might be considered ghostly or at least ethereal, they were never referred to as ghosts or spirits. Virtually nothing of what remains of Aernigh myth suggests a belief in life after death. Yet the Smoughnigh (sometimes called simply Smoughs) were often referred to as memories or vengeful spirits. They took no form, but were said to haunt the forests and the coasts, whispering evil thoughts into people's heads. Ancient tales suggest that listening to such whispers could drive someone mad or even "divorce their spirit from their body, taking the body for its own use," a rare Aernigh reference to a soul. There were several individual tales, however, of Smoughnigh helping lost travelers (especially children) escape dangerous woods, waking up coastal slumberers before high tide or even warning lone villagers of an impending storm.
There were said to be two kinds of Smoughnigh: Beith and Ciall, or Low and High. Beith Smoughnigh communicated with people through images, flashes of insight, and sudden changes in their mood or feeling. The Ciall Smoughnigh (oft called Ciallnigh) had learned mortal tongue and could actually converse with their intended victim, though no one else could hear the Ciallnigh's words.
Familiars: There are two kinds of Smoughnigh myths called Familiars. The first is a malignant animal, usually a grimling, whose body has been taken by a Smoughnigh. These animals were able to make their thoughts known to others, especially evil sorcerers, and were said to be drawn either to the wicked or the innocent, whose blood they could drink for nourishment. The other kind was a Smoughnigh that had become trapped in a person's body, unable to divorce their soul and take over. Such a person of exceptional will could then command the Smoughnigh to serve them. This service often took the form of granting wishes, though some versions said the Familiar could learn to cast magical spells from the Smoughnigh, to tell the future, or even to put the Smoughnigh into an animal, creating the other kind of familiar.