Source: Bestiary (2E), pg(s). 202f.
A heartstone is a magical periapt created from a valuable gemstone by a night hag. It grants its owner protection from disease, resistance to all manner of effects, and a number of spell-like abilities. The heartstone is also the means by which the night hag invades the dreams of sleeping mortals. The night hag uses it to gradually drain the vitality of its victim each night.
A heartstone loses its powers twenty-four hours after its owner dies, or after being separated from its owner for a similar length of time.
For additional resources, see the Meta page.