Skulduggery Pleasant Wiki

Banshees are magical beings with a scream that alerts The Dullahan to the presence of humans who have recently died. They first make an appearance in Mortal Coil, with the Banshee seen resembling an old, frail woman, though it is unknown if all Banshees appear like this. Though they are beings of magic, Banshees do not have any skills with Elemental, or Adept magic, only having the basic powers inherent to their race. It is said that if you hear a Banshee's cry the Dullahan will collect you within twenty-four hours and you will die.

It is said that humans with powerful enough spirits, may become a Banshee after they die. They are also the most powerful out of all the human spirit forms, out of ghosts and Remnants.