Obsidian Defender

Obsidian Defenders are Hostile Mobs found exclusively in the Dragon's Nest. Obsidian Defenders are hostile, inflict the Wither potion effect, and are extremely fast. They are also completely immune to all knockback unless the player is wielding a weapon with the Knockback enchantment, from which they would take significant knockback.

They are Wither Skeletons that wear a block of Obsidian on their heads, along with an Obsidian Chestplate.

Trivia

 * If you have a good weapon that can one-shot these, then this can be a good but somewhat slow source to farm obsidian. To get one stack of obsidian you would need 22-32 kills.