The Heart-Stone necklace is a possession only found upon a Druid that follows the Wither. Withers are foul-spirited evil beings that use their powers to extend their lives, corrupt the innocent and nature and generally make life difficult for good folk.
When they turn from the path of good or neutrality, they extract the heart of their sacred tree through some magical means and wear it as a charm. The tree's heart will appear to be something close to the size of a prune but hard as rock - almost like a lump of coal.
It is the Heart Stone that gives them the ability to extend their lives. Through dark magic and the sacrifice of long-lived beings (magical creatures, elves, etc.), the blood of the being is poured onto the heart-stone and revitalizes the Witch for a time depending on how many 'years' she is able to steal from her victim.
A Wither may not harm any who possess their Heart-Stone, but they will always feel its presence and will stop at nothing to recover it. For so long as the heart-stone is out of their possession, they will start to age normally again. This can be problematic for some Withers who may have stretched their lives far beyond a normal span.
A Wither (druid) of a Hundred years who has stolen herself down to forty, will return to her normal age within a few days. A Wither of only sixty years but who has stolen herself down to forty has a few weeks as the transformation is not as drastic.
While without her Heart-Stone, she will be unable to disguise her appearance, nor use the blood of the innocent to heal her wounds.