A Bean nighe is a Scottish fairy or death-spirit that is similiar to the Irish Banshee - said to haunt rivers and streams a Bean nighe is an omen of death and is said to either be the spirit of the dead, a malevolent fairy or a form of an ancient Celtic war-goddess known as Morrigan.
Legend
According to legend a Bean nighe takes the form of an old hag dressed in green who sits by rivers and streams washing bloodstained clothing, any mortal who passes this spectre will be doomed to die or have disaster befall them in a short while (such as the death of a loved one).
Although often hideous a Bean nighe may on rare occassions take the form of an attractive maiden but doesn't actively seduce mortals, indeed in both forms the spirit doesn't truly interact with humans - it is said that their presence alone is sufficient to cause death and destruction in those that encounter them.
Often seen as a type of fairy of spirit there are also some legends that state a Bean nighe is created when a woman dies in childbirth, her spirit is doomed to haunt the world until her life would of normally ended (in the medieval world, where folklore was common, childbirth was a dangerous and often fatal experience thus it was only natural such tales would be woven).
In the modern world a Bean nighe is normally regarded as little more than an alter-ego of the Irish Banshee but this is technically false as they are two different creatures despite their similiarities.