The Gleed! A mob that spawns ONLY in player-made mines,digs out a 3x3x3 opening in your mine, seals it with Cobblestone, places down a chest, puts cobblestone in it in a sideways H formation, then, when you open the chest, starts to deal 5 hearts of damage each second, until you die, OR kill him somehow, because it has 50 hearts of health. (More mobs planned!)