An underground forested biome, where you can get wood at the cavern level and grow trees. Herbs and strange plants would grow there, bosses would spawn... basically an underground alternative to the surface forest, but closer to lava than sun.
Also an alternative to the Hallow, wherein instead of everything being happy and pink rainbows, it's all serious and solemn and reverent. Stone is brown tinged white, and instead of glowing in the dark, enemies are wispy and shed light, making them harder to see. Also instead of crystals you have geodes, that do not glow but sparkle when light is brought close, which is often