This biome would spawn very rarely in the nether, It would consist mostly of wither related items, Wither skeletons would be the main threat in this biome. However the thing that makes this biome so dangerous is the blocks itself. Rather than lava it would be a new liquid known as "Soul Lava.” This soul lava would be twice as dangerous as normal lava. Not only would it burn you. It would also give you the wither effect. You could collect the liquid like any other but it would be used for one main purpose, Brewing. If you put the bucket in a brewing stand with akward potions it will brew wither potions. Now this liquid would drip from the ceilings and would have a black color to it. Now for the rest of the biome, A new structure would spawn called "The Wither Alter" It would basically be an altar with a failed attempt at summoning the wither. I feel like this would help the player would learn how to summon it without googling it. This biome would be extremely rare but be very large when you find it.