Mob Spawners, although a vanilla feature, are generated more frequently by Deadly World. They can spawn in any open spot below level 62, and have a chance of turning into Swarm Spawners or Brutal Spawners. Spawners have a chance to spawn with a loot chest, and can also be armoured. When armoured, the Spawner will be coated in obsidian, making it harder to destroy. There is also a chance of it being a trick armoured spawner, meaning that it is just a harmless chest covered in obsidian.
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