Dungeons.
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The seemingly simple concept of a dungeon might change depending on who you ask, but the general idea stays the same: Some sort of underground room or compound. A labyrinth, a torture chamber, a boss arena...
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But what about a world?

An entire ecosystem of an underground realm. That's where our protagonist awakes.
Bound in a body made of earth and mud, follow our protagonist as he learns to survive and maybe even thrive as an elemental in a labyrinth where survival of the fittest is the only rule.