Quicksilver, Speedster

The last clause is the one that turns a fair combat creature into an engine: while this is tapped, your whole hand gains flash. So the license matters more than the body. Cast it during your turn, and it swings with double strike and haste; the instant it taps to attack, every sorcery-speed spell in your hand becomes an instant, letting you develop through combat and then empty your hand in response to whatever your opponents try. The design leans into a rhythm blue-red already wants: react, then punish. There is a wrinkle in the timing, though. The flash-granting clause only comes online once the creature is actually tapped, so flashing it in during an opponent's end step gives you a haste attacker next turn but no hand-wide flash until you commit it to combat. Its 3/4 body is deliberately unimposing for five mana, and the pull between wanting to attack (to pressure life totals with double strike) and wanting to stay tapped (to keep the flash license live) resolves itself, since attacking is what taps it in the first place. A creature that rewrites the timing rules for your entire hand every combat is doing far more than its numbers suggest, and the speedster framing (a hero who bends time by moving fast enough) is one of the tighter mechanic-to-flavor marriages in the crossover space.