This is the hero's spirit boss fight from weapons of war phase 4 it is the deceased soul of a former hero that is looking for another hero to be bested in combat against it is also one of the rarest mobs in fabric modding you could ever find, it is not structure based and is fairly powerful, the way to start the boss fight is to attack the spirit and then you need to fight to the death. it drops a heroic ingot on death.