You can obtain the weapon normally through the course of the game if you try the bonus dungeon (It's 100 floors of randomly generated hell, don't do it!), but even then your chances of finding it are slim-to-none on account of the random generating-ness. Also, it might be a treasure you can't even take out of the dungeon, I can't recall (I just remember using it on the boss at the bottom).
*Psst* Look at edit above. I'm too lazy to copy/paste >.>
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