To get this weapon you'll need a sword that can break walls. In the Mystic Caves, after you get past the first gate that you had to unlock, you'll reach a passage that' divided by a pit with a switch on the floor. Drop into the pit, and at one end you'll
see a breakable section of wall. Break this wall to expose a weapon rack. Break the rack and you'll be able to pick up the Glowing Sword, an unbreakable secondary weapon that does a ridiculous amount of damage, but will gradually cause increasing damage to you each time you use it.