The Bonebreaker is a mace in Final Fantasy XII. It is one of two new maces added in the Zodiac versions, the other being the Bone of Byblos. While it is classified as a mace, it uses the pierce formula, which does not take into consideration the user's or target's stats in its damage calculation.
Bonebreaker is one of three secret weapon added in the Zodiac versions, alongside the Seitengrat and Great Trango. The secret weapons are extremely unlikely to be obtained without knowledge of their existence, and do not require licenses to use, and therefore can be used regardless of class. Unlike the other two, Bonebreaker has a visible model, and has a unique model (the only other added weapon to have a unique model is the other added mace).
Unlike the Great Trango and Seitengrat, which have exceptionally high stats, the Bonebreaker is relatively weak. Its utility is in that it is guaranteed to inflict the Disease and Sap statuses on-hit so long as the enemy is not immune. These statuses in tandem are lethal, because unless the statuses are cured, the afflicted target's HP will drain because of Sap, and offsetting the damage with healing is not possible because of Disease.