Codexus is an optional boss found in the Library. Near the end of the game, you will head to the top of the Spelltower and be shown cracks forming around Ikenfell. One will form inside the Library and knock Codexus off the shelf. Back track to the Library basement to challenge Codexus. You can activate the Instant Victory in the options and then select Instant Victory when it is your turn to defeat the boss.