He reshaped the Mire into a bowl instead. His will sank the basin floor thirty meters below the elevation of the magma burrow's exit on one side and thirty more below the threshold of the Verdant Expanse on the other, abyssal rock rising sheer on all four walls of the sunken pit and capping the whole biome overhead in the same impenetrable stone. The poison was heavier than air. It settled. It stayed where he wanted it.
Clean air lived above the rim of the bowl, out of reach of anything walking the floor. Anything flying had no better option. The biome's ceiling was that same abyssal rock pressed flush against the dungeon cap, and the full vertical airspace beneath it was the vapor layer itself. A flyer climbing to escape the floor spores would only rise into thicker poison and then into stone that would not yield.
The dungeon's baseline rule that biome atmospheres did not bleed across their own geometry did the rest.
