This golden key unlocks the underground kingdom.
But to make a golden key, we must first gather spider legs, body fluid, claws, and fangs.
There are Blackrock Spiders and Scorching Spiders south of the Flame Altar. We can get all kinds of spider body parts from them. It'll be fun! Well, maybe not fun. Maybe just gross and shudder-worthy.
According to this book, the underground kingdom was once a system of caves occupied by giant monster spiders. The Goblins discovered it while digging around the volcano.
Goblins used the power of a dark crystal to build an underground kingdom.
If we cover ourselves in spider legs, body fluid, claws, and fangs, we can fool the monster spider into believing that we're spiders, too, and he'll let us use the key. They have horrible eyesight, so I think we can pull this off.