The requirements to unlock the Dimension's Hasp extra dungeon depends on the ending you have earned. In the bad (Finale) ending, after the unbeatable Boss at the start of the Finale, see DeRosso in the Vampire Castle. In the good (The End) ending, speak with Yulyana after the initial Bosses you are supposed to defeat. The Dimension's Hasp extra dungeon can be found in the center of the crescent-shaped island in the northeast corner of the map. The bonus Boss is hidden at the end of the dungeon on B10F. Once there, move east of the final chest (with the Durandal inside), and then go south.