A place for the wither to spawn

A structure , dimension or biome for the wither to spawn because it would possibly be able to add lore and effect to the game and maybe if it was a structure it could be found in the soul sand valley or if it was a biome it could be a new biome in the Nether or it could even just be in a new room in 1/2 of nether fortresses where it would be it almost ready to be spawned as in one skull could be missing.