THE DARK SWORD WEAPON!!

Hi, I'm John as eloisa10 lvl76 in DARK SWORD.I'm playing Hard, I eliminated lots of boss in Dark Dungeon mode and from now on I'm at lvl 20 not so far but I've never been acquire Dark Weapon.. How and when can I bet this strongest unique weapon? I'm still hoping that I will equiped! this sword right a way.. :)