Cave Biome: Nether Corruption

The Nether Corruption cave biome would turn most rock types of the affected area into netherrack and all water into lava, while causing the space within to be treated as the nether to allow nether mobs to spawn in their non-zombified states. Within the corrupted space, any nether biome can generate over it as if in the nether, even multiple nether biomes at once.

Of course, as a unique cave biome, it wouldn’t just be the nether but in the overworld and wouldn’t spawn things like full nether structures so as to not make going to the nether pointless. Rather, smaller ruins or chunks of such structures that got caught in the implied incursion would appear within the corrupted caves and would also contain at least one ruined portal structure as if the corruption had spread from it. It could also contain nether ores, though not netherite for the same reason as the structures (if so, only very rarely).

The Nether Corruption cave biome itself would be rare to find and only generate at or below Y0 within the deepslate layer.