The High Disciple lives on the 5th island and wants to know if you are the strong human being mentionned in the prophecy which says that one shall stand against Mathieu, the most powerful seraph, and defeat him. You have to talk to him and he will teleport you on Mathieu's Island. From there, you need to talk to the Disciple in order to get the quests infos. You need to kill 30 Fallen Angels and report to him to show that you"re strong enough to defeat Mathieu. He will give you a key to open the door behind him and you can finally face Mathieu, the god of Althea.
