The Highland name for the Aurora Borealis. Mackenzie, in Scottish Folk-Lore and Folk Life (p.222), gives a good account of the tradition about the Fir Chlis, distinguishing their 'everlasting battle' from the more hurtful activities of the sluagh. He himself was told of the 'Nimble Men' engaging in fights between the clans of two chiefs, rivals for the possession of a fairy lady. The bright red sky sometimes seen beneath the moving lights of the aurora is sometimes called 'the pool of blood'. J.G.Campbell, in his Superstitions of the Highlands (p.200), says that the blood of the wounded, falling to the earth and becoming congealed, forms the coloured stones called 'blood stones', known in the Hebrides also by the name of 'fuil siochaire' ('fairy blood'). In Ireland, according to William Allingham's poem 'The Fairies', the spirits composing the aurora are more truly 'Merry Dancers', for the old fairy king is described as:

Going up with music on cold starry nights

To feast with the Queen of the gay Northern Lights.

According to Lewis Spence in The Fairy Tradition (p.58), the Fir Chlis were supposed to be those fallen angels whose fall was arrested before they reached the earth. This Christian theory of the origin of fairies was particularly prevalent in the Highlands, for almost every Highlander was a theologian. The Suffolk name for the Northern Lights is Perry Dancers.