Fantasy greatsword design

While wielding this shortsword and a moon fang, you can expend 4 charges as an action to cause both items to magically meld together for one minute to create a heaven piercer. For the next minute, you also have proficiency with the greatsword. At the end of the duration, the heaven piercer splits back into this shortsword and a moon fang. This property can't be used again until the next dawn.