Field of Bone (CR 6)

This supernatural hazard usually plagues those who trespass on old battlefields still littered with the bones of soldiers who have never been laid to proper rest. These 30-foot-radius patches of strewn bones are considered difficult terrain, and they spring to a foul mockery of life 1 round after a living creature enters the area, causing 1d6 skeletons to animate and attack, as if subject to an animate dead spell. A field of bone can animate up to 24 skeletons in this manner from any single instance of trespassing (regardless of how many living creatures trespass into the area at once), at a rate of 1d6 skeletons per round. The skeletons continue to animate until all are destroyed, all living creatures leave the area, or the field of bone reaches its animation limit, whichever of these conditions comes first.