Most commonly associated with the English bogs and moors, these phantom lights are said to lead travellers astray, and are usually malevolent. They appear mostly with a blue-ish gleam but red is also mentioned. Pretty, but dangerous.
These lights are known by various names in many other countries around the world. Here in Australia they are called 'min min' lights, and, as like their English counterparts, tend to like to lead travellers astray. They are seen most commonly in the Outback, and it is said that if you turn to try and follow them, you will never been seen again.
As is a common theme with the unexplained, the folklore accounts for them as lost souls, or souls turned away from heaven and hell.
Scientists believe the phenomenon is likely the ignition of gas marshes, or bioluminescent algae or fungus, glowing because of a chemical process. (Not as much fun as a glowing ghostly soul!)
In The Diabolus Chronicles we first meet will-o'-the-wisp in book two, The Verderer, where a cluster of them nearly run Silas and Pitch off the road in their attempts to escape a monster in the Forest of Dean. But it is in book six, The Herlequin, that they are seen for more than a fleeting glance. Scarlet goes against popular folklore in that they are not the least bit mean or nasty! And this rainbow coloured ball of ghostly gas has become one of the most popular characters in the series.