New underground boss

A boss found deep underground, below y=20, somewhere within 200 blocks of the world spawn. It spawns in a large cavern, with cobblestone in the walls, and in the caves surrounding it. The cobblestone is a sign that you're close to it. It has 150hp, and looks at first, like a large piece of cobblestone. When you get near it, it unfolds into a hulking beast, with lava within it, visible through the eyes, mouth, and various holes in the body. When it gets down to 60hp, it goes back up to 75hp, and much more of its body is lava. From that point on it sets you on fire when it hits you. When killed, it drops a new item. Not sure what the item could do but possibily: When put in a crafting table with a beacon, it becomes a death beacon, and instead deals minor damage over time to hostile mobs in its range.