Light Sword
The light sword is the most powerful secondary weapon in the game. When in the mystic caves you come upon a door that you must unlock via switch. Directly after that door there is a drop in the floor. Drop down and break the left wall and break the rack to pick up the light sword. As with all secondary weapons it is unbreakable, but if you throw it you lose it for good. The light sword has maximum damage and maximum attack rate.