This is a chest that is very similar to a mimic in that it is actually a living thing pretending to be a treasure chest.  It configures itself to be both locked and trapped at a relatively low DC.  When a character attempts to disarm and unlock it, the result will be either a sneering acknowledgement if successful (possibly with a small amount of treasure) or VICIOUS MOCKERY(the bardic spell) if unsuccessful.  The chest will then close and reconfigure itself into a more difficult DC and demand the character try again.  If the character successfully opens the chest three times they receive a wondrous magic item.  If they fail three times, something very very bad happens to the character at the DM's discretion.