Lightning Serpent

Pay one mana, get a 2/1 with haste and trample. Pay more, and you scale the body straight up: the X loads on as power, the trample carries the overflow, and the haste delivers it the turn it lands. Then it dies. The end-step sacrifice is the whole transaction laid bare: this is a burn spell wearing a creature's clothes, a Fireball that hits a player through blockers rather than over them. The counters frame matters more than it looks, because they make the damage a permanent on the board for one combat step, which means it can be doubled, it can answer a midsize threat in combat before swinging the rest at face, and it cares about anything that pumps power. That single window (your turn, one attack, gone by your end step) is the price you pay for the X-spell efficiency, and it is what keeps the card from being a clean removal-and-reach machine. You commit the mana, you commit to attacking, and you accept that there is no board left afterward. The lineage here is the long red tradition of turning a creature into a one-shot damage delivery system: a haste body that exists to connect once and convert mana into the largest possible hit, with trample ensuring a chump block only taxes it rather than stops it.