First, get the Sol Blade from the Apollo Sanctum and complete the game. Save the game when prompted and load the cleared saved game file. Starting at Tonfon, sail west until you come across a small island surrounded by rocks. Wait until a whirlpool appears and sail into it to reach the other side of the rocks with the island. Land on the island, go inside, and select the door. After some dialog, open your inventory and use the Sol Blade by selecting it. The maze is where the Summon Catastrophe and Ancient Devil Boss are located.