You have obtained one of the most recognizable weapons in the Souls franchise. The Moonlight Greatsword. This weapon is of immense quality and can act as a catalyst for most magic, and any magic used with it will have its efficiency augmented. By feeding it mana, or stamina should you have no mana left, you can shoot a beam of magic that can level buildings (provided you supply enough energy). The sword can also inflict frostbite by choice of its user.
(The appearance of Moonlight can be chosen by the user when buying.)