I made a little research out of curiosity and found out what it is:
The BANSHEE ("woman of the s'idhe" or "woman of the fairy mounds") is a FEMALE SPIRIT in Irish mythology, usually seen as an OMEN OF DEATH AND A MESSENGER FROM THE OTHERWORLD.
In legend, a banshee is a fairy woman who begins to wail if someone is about to die. In Scottish Gaelic mythology she is known as the bean s`ith or bean-nighe and is seen washing the blood-stained clothes or armour of those who are about to die. Alleged sightings of banshees have been reported as recently as 1948. Similar beings are also found inWelsh, Norse and American folklore.
The story of the banshee began as a fairy woman keening at the death of important personages. In later stories, the appearance of the banshee could foretell death. Banshees were said to appear for particular Irish families, though which families made it onto this list varied depending on who was telling the story. Stories of banshees were also prevalent in the West Highlands of Scotland.
The banshee can appear in a variety of guises. Most often she appears as an ugly, frightening hag, but she can also appear as a stunningly beautiful woman of any age that suits her. In some tales, the figure who first appears to be a "banshee" is later revealed to be the Irish battle goddess, the Morr'igan.
The Morrigan - Celtic Goddess
Although not always seen, her mourning call is heard, usually at night when someone is about to die and usually around woods. In 1437, King James I of Scotland was approached by an Irish seer who was later identified as a banshee who foretold his murder at the instigation of the Earl of Atholl. There are records of several prophets believed to be incarnate banshees attending the great houses of Ireland and the courts of local Irish kings.
In some parts of Leinster, she is referred to as the bean chaointe (keening woman) whose wail can be so piercing that it shatters glass. Her keen can be experienced as a "low, pleasant singing" or as "the sound of two boards being struck together". Sometimes also as "a thin, screeching sound somewhere between the wail of a woman and the moan of an owl".
The banshee may also appear in a variety of other forms, such as that of a hooded crow, stoat, hare and weasel - animals associated in Ireland with witchcraft.
In later versions, the banshee might appear before the death and warn the family by wailing. When several banshees appeared at once, it indicated the death of someone great or holy. The tales sometimes recounted that the woman, though called a fairy, was a ghost, often of a specific murdered woman, or a mother who died in childbirth.
Banshees are frequently described as dressed in white or grey, often having long, pale hair which they brush with a silver comb. This comb detail is also related to the centuries-old traditional romantic Irish story that, if you ever see a comb lying on the ground in Ireland, you must never pick it up, or the banshees (or mermaids - stories vary), having placed it there to lure unsuspecting humans, will spirit such gullible humans away. Other stories portray banshees as dressed in green, red, or black with a grey cloak.
Another version of what a banshee is and does.
Credit: pagan-magic.blogspot.com
