Once you've reached Lv 30, go back to Ellinia's Magic Library, which is located at the highest point of Ellinia. Go inside, talk to Grendel the Really Old. He'll give you a letter and you've to pass it to the job advancement instructor located at the north of Ellinia. After giving the letter, you'll be given an errand to collect 30 Dark Marbles by defeating the monsters inside. Be careful to stock your potions up before you start collecting the 30 Dark Marbles. Once you've collected 30 Dark Marbles, talk to the instructor, and he'll award you with a Proof of a Hero. Go back to Ellinia's Magic Library, and talk to Grendel the Really Old once again, where then you'll advance as the Cleric. Requirement: Level 30