Inferno

Six damage to every creature and every player, the caster included, at instant speed: the symmetry is the whole design idea. Most red sweepers of this early era stayed cheap by hitting only creatures, but this one priced its self-inflicted clause into the rate. Seven mana buys a Fireball that cannot miss, but it also points the muzzle back at your own face. The damage to each player is what turns a control valve into a finisher: fire it when an opponent sits at six or less and you have a one-card kill that happens to clear the table on the way out, an early example of red treating the board sweep and the burn-to-the-dome as one spell rather than two jobs. The instant-speed window matters more than the bulk number suggests; it lets you wait for a profitable combat or an over-committed board before pulling the trigger instead of tipping your hand on your own turn. It is a blunt instrument by modern standards, and the life cost makes it something you set up around rather than reach for freely, but the shape it sketched (red as the color that will burn everything, the player firing the spell very much included) has echoed through every symmetrical damage effect the color has printed since.