Instructions
Progress through the first nine levels of the Midlight's Deep dungeon. At each level, you must locate the exit tile in the darkness before defeating the last enemy on the battlefield. The specific tile that will be the exit tile in your playthrough is random, but you will know you have found the right tile when the text "Found a passage leading deeper" appears on the screen.
Enter the 10th level of the Midlight's Deep dungeon. Once you enter this level, battle against Elidibus will be initiated, and Byblos will join your party.
Move your party of characters up the staircase toward Elidibus, defeating the Reavers he summoned along the way.
Fight Elidibus using a combination of your strongest attacks, whatever they may be. Attack Elidibus while he is charging up to use his Zodiark summon, as he takes the most damage while charging.
Continue to battle Elidibus, making sure that Byblos stays alive during the fight. Should Byblos get killed by Elidibus, use either an Angel Ring or a Chantage to revive him.
Defeat Elidibus, and Byblos will offer to join your party. Select the "Allow" option to allow him to join, and he will become a playable character.