Bone Spirits are ghosts which have existed on Earth for countless ages, so long they have forgotten the last vestiges of their former earthly lives. For one reason or another they stayed behind on the Earthly plane, perhaps seeking closure for an Earthly injustice of some kind. But time wears on, and those who live upon the Earth have short memories.
They roam the landscape, slowly counting the ever lasting years, watching nations raise and fall and taunting them with the countless transient lives of human beings which seem to flash before them in the blink of an eye. Who can say what insanity goes through the mind of a being trapped on this Earthly plane who's life ended before the advent of language? How can such a being find closure when all it knew in life lived, died, and turned to rot so long ago as to be counted on a geologic scale?
Truly it is not those who pass on to parts unknown which are cursed, but those who are left behind, as few of us possess the fortitude to withstand infinity.