Aegar, the Freezing Flame

The word "excess" is doing all the work here. Damage normally stops mattering the instant a creature's toughness is exceeded: point five at a 3/3 and the last two vanish into nothing. This design salvages that overkill and pays you for it, turning the burn player's instinctive habit (throwing a big red spell at a small blocker, swinging an oversized attacker into a chump) into a draw engine. The condition is specific in a way that shapes deckbuilding around a Giant-Wizard-spells triad: any of those three sources landing damage on an opponent's creature or planeswalker before it dies arms the trigger. That means it rewards big splashy burn and oversized attackers rather than precise removal, because the whole point is to hit for more than you need. A Fireball that deals ten to a two-toughness creature is now cantripping eight times over conceptually, though the trigger fires once per lethal event; the incentive is to swing the axe hard rather than clean. The 3/3 body counts itself as a qualifying attacker, so it participates in its own engine, and instant-speed burn lets the whole thing fire on an opponent's turn during combat. It is a rare payoff for inefficiency: most cards punish you for overcommitting resources to a single kill, and this one hands you a card for doing exactly that.