The Great EZI is a super-boss in the PS3 version of Eternal Sonata. He is a (unusual type of) deity that is worshipped by several people. There are statues and obtainable items that are related to him (which are strange, useless, or even dangerous).
In the game, Allegrato wanted to investigate the Church of EZI just to know what he's like. March used the EZI pass and said whatever was on it and it granted them passage. It is never recommended to enter the bonus dungeon while under level 75. There were stuff that would seem good, but would actually harm the enviroment towards Ritardando. Later, Allegrato, Polka, Salsa, and March saw a few followers munbling around the EZI statue. When March almost touched it, EZI grew into a giantic size and went after them, thus engaging into a challenging battle. Afterwards, as they managed to escape, its unknown whether EZI himself was dead or that it was just a statue.