Pain Kami

Stored burn in a creature costume, deployed a turn early and fired the moment a target shows up. The whole point is to die on command: the body rarely wants to attack or block, because its reason for existing is to convert untapped mana into precise creature damage at the instant you need it. That structure is what makes it sturdier than one-shot burn. It sits on the board as a small threat opponents must respect, and it answers their removal in kind, since when they go to kill it you can sacrifice it in response and still land the shot. The Spirit still earns its keep when it has to: a chump in front of an attacker, fodder for a sacrifice outlet, or a two-point swing before you cash it in. This is an early-era design instinct that loved creatures doubling as spells, the burn effect wearing a creature shell so it survives the cards that answer instants. The catch lives in the open-ended X: the damage is only as large as your untapped mana, so late it snipes nearly anything and early it is a glorified Shock, and you pay full price plus the creature itself every time you pull the trigger.