The Boneyard Kingdom is the land ruled by Skeleton Princess, according to Adam Muto.[1] Little is known about the kingdom as it only has a small appearance in the episode "Princess Monster Wife", In the episode, it shows Skeleton Princess lying on a bed within a room that seems to be inside of a tree. The room is filled with small candles (and possibly a candelabra).
In the comics, it is explored much more thoroughly, as it appears prominently in The Four Castles, where the outside of it is shown.