A biome with only hostile mobs spawning.

Okay so the Overworld is usually safe and kinda colorful but what about a Biome where black clouds cover it at all times and you need to place torches down to prevent getting attacked and having a slight blindness effect when you enter it. The Biome could contain poisonous water and on ground dungeons with some mobs that hit extremely hard without at least Iron Armour. The Biome is more rare than a Mesa Biome or Extreme Hills and has more of a chance to generate near the player on Halloween (October 31ist). The only name i can think for the Biome is "Everdark" or "Murk"