Unique power (on hit) is an on-hit: cast spell item property that serves as a generic placeholder for custom behavior that is to be triggered when the item in question makes contact in combat. It is sometimes used when the nature of the custom behavior is not supposed to be immediately obvious to the player, but it is also used when the builder feels that defining a new spell for the behavior is not worth the effort (particularly since defining new spells necessitates a hak pak and talk table). The details of what this property does are completely dependent upon the module being played.