Portents warn of a Great Tidal Bore—a calamity that strikes once every decade wherein monsters surge up from the depths of Lune's dungeon. The signs have been growing stronger day by day...but the easygoing Ryo never notices, immersed as he is in his hobbies. And then, it finally happens while he's holed up in the library seeking more knowledge about alchemy!

The guild and its adventurers fight off tens of thousands of goblins. Just as they find themselves exhausted by the seemingly never-ending battle, the goblin king appears. How will Abel and his comrades handle this unforeseen enemy...?! Find out in the fourth manga volume of the adventure featuring the strongest water magician who just wants to live life at his own pace!