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 back to the Burning Island Cave and go to the bottom of the cave. Find the previously locked door and select it. After some dialog, open your inventory and use the Sol Blade by selecting it. The room beyond is where the Summon Daedalus and the Ogre Titans Bosses are located.