The Book of Lyl
Shadow Spirits are undead usually crafted from the remnant emotional energy of good people who have undergone hardship leading to their deaths. Shadow Spirits spread by consuming the life force of their victims. When someone dies at the hands of a Shadow Spirit, their own shadow takes on unlife. Should that person be brought back to life through supernatural means, they no longer have their own shadow, and the newly animated Shadow Spirit seeks them out to bring them back to the grave.
First encountered by the adventurers in The Lost Tomb of Kha’Em.