Tharkos of the Steel Eye is a minor baron of Hell's outer reaches. He is the infernal patron of servility and slavery, and delights in the groveling self-abasement of other beings (which he encourages in any way possible).
Among his many other devilish abilities, Tharkos possesses the unusual power of gifting his living heart to a mortal creature. Such a gift always requires an exceptional favor, rendered in advance, though Tharkos will often demand additional services to be determined at a later time in order to keep that mortal under his power in an ongoing way. Tharkos can gift his heart to a mortal no more than once every seven years, for it takes this long to regrow.
If the subject is subsequently restored to life in some manner, the mark of Tharkos will once again be in its original position. There is no known method of permanently removing the Mark, other than permanent removal of both arms (if one is removed, the Mark will appear on the other shoulder the first time the moon rises).
Any number of beings can be bound by a single Bloodstone of Tharkos as long as it remains intact. If it is ever destroyed, all those previously bound remain bound, but Tharkos will not replace it.