When the head is seperated from the body, the neck stump weeps baleful blue flames which are mostly illusory. Or made of freaky soul stuff that lets the head perceive things (hear, smell, see and even taste) without the head attached.
bluecap is a mythical fairy or ghost in English folklore that inhabits mines and appears as a small blue flame. If miners treat them with respect, the bluecaps lead them to rich deposits of minerals. Like knockers or kobolds, bluecaps can also forewarn miners of cave-ins. They are mostly associated with the Anglo-Scottish borders.