The Pyromaniac is a boss from the Nether Expansion idea pack. It spawns anywhere in the Nether, but about 1/4 as rarely as Witches do in darkness. It has a total of 250 HP (125 hearts). The Pyromaniac is a relatively hard boss, but not nearly as deadly as the Wither King.
In the first phase of the battle, it will set all the blocks in a 3x3 radius around it on fire. This makes it rather dangerous. It will run around and hit opponents with a Netherrack Sword. Remember that the fire caused by it will never go out unless cooled by water or water bottles! It does damage in this stage.
The second phase of the battle is triggered when the Pyromaniac hits 125 health. All the fire will be extinguished, but it will start doing damage. In addition, it will start wielding its deadliest weapon, the Heat Ray. This can set opponents and blocks on fire and even smelt blocks.