The giant eyeball monster was protecting a magic mirror that serves as a gate to the goblin realm. With goblins streaming to their location from every direction (but the mirror that is) Goblin Slayer comes up with a plan to deal with them.
First, the way in is barricaded to slow them down. Then Dwarf and Elf help GS attack the goblins who come as the Lizardman and Priestess dismount the mirror from the wall and point it upward. They all huddle beneath it and the Dwarf brings the entire roof of the structure down.
The resulting damage flattens all of the remaining goblins, while the rubble falls harmlessly through the mirror gate. All in all, a neat little victory, but when he reports back to the Sword Maiden, GS deduces that she knew all about what was going on down there.
She confirms that the white alligator was her familiar and that the goblins beneath the city were allowed to roam not just because she feared being seen as weak or overreacting, but to demonstrate to the people how the goblins were a serious threat, owing to her own personal torment by their grubby hands.
The Sword Maiden carries trauma the GS will never be able to save her from, but he’s always been a highly practical man, and so he bids her farewell with the promise that if she needs more goblins slain for her, he will come when summoned. Until then, he’s headed back home to make ice cream.
You heard right: the man once known only for slaying goblins intends to do something other than slay dragons. Perhaps like those anime school clubs that do more than one thing, a name change is in order: Ice Cream-Making Goblin Slayer? Doesn’t exactly roll off the tongue…