Vedalken Ghoul

Most penalty-on-attack creatures price their threat into combat damage: the defender pays only if the attacker connects, so any cheap chump block resets the danger to zero. The wrinkle here is the trigger word. The four life is lost the instant this creature becomes blocked, before damage, before anything dies. Putting a body in the way does not duck the cost; it costs four life just to stop a single point of damage, and the blocker still gets to kill the attacker in the process. That inverts the defender's usual math. Normally a 1/1 attacker is a free block: trade nothing, take nothing, profit. Against this, the cheapest available blocker bleeds you for four to trade with a 1/1, while waving it through costs only one. The least-bad line is often to take the point and keep the life, which means the point lands no matter what. The toughness is the price the gold cost pays for that nastiness: a single 1/1 only ever forces the question once, since it dies the moment a blocker deals it lethal damage, and any removal spell or a defender content to take one point ends the pressure entirely. What it leaves behind is a threat that is almost entirely psychological, a two-mana dare that turns one combat step into a question with no clean answer, where blocking and not blocking both cost real life and the creature itself rarely survives to ask twice.