The Heart Box appears to be a small, wooden cage, painted red. (those of a macabre mind may be tempted to make assumptions about the nature of the paint, but a series of very complete tests by the last holder of the Box proved that the paint was ordinary, though attempts to carbon date it were inconclusive. The wood, incidentally, is rose.) The Box is approximately the right size to hold a human heart, though there is no record of it having been used for this purpose, nor any hint that this was its intent. Rather, the name Heart Box seems to have come about because of the Box's size and colour.
The top of the Box is hinged, and held closed by a golden clasp. Under normal conditions, it can only be opened by its rightful owner. There are apparently exceptions to this, but none are known for certain. Some rumored exceptions follow:
Allegedly, if the owner of the Box is male and was born by C-section, his mother can open the Box.
Allegedly, the owner's spouse can open the Box by picking the clasp with a sewing needle. This method is known to have been used at least once.
Allegedly, the owner's second son can tie a lock of his own hair around one of the Box's bars, and by doing so will become its owner, allowing him to then open the Box at will.
Allegedly, during a waning gibbous moon, a paring knife applied correctly can pry open the Box's clasp.
Other rumors exist, but the above have each been told repeatedly through history, and so are the most likely to be true.
As for the Box's actual function, once opened it is able to capture and contain a single human soul; that of its rightful owner. This cannot be done to any person other than the owner, or against the will of the one giving up their soul. The exact method for filling the Box is unknown, but seems to work on a mental stimulus. Once the Box is filled, its inside will glow with a faint blue light.
The one whose soul rests within the Box will be left with all of their mental faculties intact, but devoid of emotion or intuition, and incapable of experiencing rage, fear, lust, etc. They will still feel pain and pleasure but will not react to them. All of their actions will be determined entirely by the logical mind. This has the benefit of shielding the person in question from encounters which would otherwise be maddeningly terrifying, enraging, beguiling, etc.
The other benefit of the Box's ministrations is tenuous immortality. So long as the Box remains intact and sealed, the one whose soul it contains will be unable to die, no matter how gravely injured. Again, there are exceptions to this rule, but they are not adequately known.
If the Box is opened, any soul contained within it will return instantly to its owner, leaving the Box empty. If the box is smashed, the soul's owner will die. The exact fate of the soul from that point is unknown.
Interestingly, though there are numerous reports of individuals storing their souls in the Box in order to reap the benefits of such, these individuals have never shown any subsequent inclination to regain their souls or indeed to protect them from harm in any way. It is suspected that their loss of fear of death is responsible for this.