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The Banshee is most commonly visualised as a female spirit who wails in the night to foretell disaster, either to an individual family or more generally. The tradition is the strongest in Ireland but many places with Celtic survivals have a variant of the Banshee.
From most descriptions, the Banshee appears dressed in green, perhaps wearing a grey cloak over her wizened body, with long streaming hair and eyes red from weeping. She is depicted both as a young woman and as a hag. Although often described as above, her most common characteristic is mournful wailing during which she heard but not seen.
Many old, long established families had a Banshee who warned of impending death. She would appear in the area around the ancestral house in the middle of the night and cry in mournful lamentation. In this she fulfils the role of other supernatural creatures such as the radiant boy, and the phantom drummer or piper, also portents of death for particular 'noble' families.
In some stories the Banshee still wails even though the family member who has died is living abroad, far from their ancestral seat. Traditionally in Ireland, she is seen as the spirit of one of the earlier generations of the family who has taken the task to warn of impending doom down the centuries.
The actual term Banshee is derived from the Gaelic Bean Sidhe (Bean Si), which means 'woman of the fairy'. In Scotland the Bean Nighe or washerwoman at the ford fulfils the same characteristics, washing the clothes of those who are about to die. In Wales the role is taken by the Gwarach-y-rhibyn, a hideous hag who also haunts old Welsh families.