Fireball

Every variable-cost burn spell that came later was either built on this template or deliberately broke away from it, which is the clearest sign of where the category began. Two distinct jobs run at once here: the spell scales with the mana available (the X chassis that would later define Comet Storm, Crackle with Power, and a dozen others), and it taxes multi-target use with an additional generic mana per extra target. The tax is the part later designers mostly abandoned. Disintegrate dropped it; Banefire dropped it and added uncounterability; Comet Storm replaced even division with free distribution at a steeper rate. Every one of those reads as a deliberate rewrite of a Fireball constraint, which is how you can tell Fireball was the reference point. The "divided evenly, rounded down" math is the other fingerprint: split across three targets at X=5, each takes 1, and the remaining 2 damage vanishes into the rounding. That quirk has been litigated in judge calls for decades and is the reason later divided-damage spells specified "as you choose" instead. What sits on the card today is, almost line for line, what was printed at the game's origin; the rules templating has been cleaned up over the years, but the design has never needed a functional revision. It is the rare early-set card whose job was to establish a category, and the category is still named after it.