Celestial Crusader

A flash anthem is already a contradiction in slots: anthem effects want to be on the battlefield before combat, while flash wants to ambush. Pairing both with split second is the wrinkle that resolves the tension. Flashed in mid-combat or in response to a threatened attack, this drops a 2/2 flier that simultaneously pumps your whole white board, and because split second locks the stack the instant the spell goes on it, the opponent cannot respond to this spell with removal, a counterspell, or an instant-speed pump while it sits unresolved. The result is closer to a one-shot combat blowout dressed as a body: declare blocks, flash it in, and the +1/+1 retroactively reshapes the math without giving the opponent a window to react to the anthem entering. Once it resolves, of course, priority returns and an opponent with mana up can still kill the Spirit before damage, which is the real cost of leaning on it: the boost only holds if the body survives, and the anthem touches other white creatures, so it lifts the team but never itself. Split second was a rare mechanic, used mostly to make the most disruption-proof versions of effects that already existed: counterspells, removal, tutors. Stapling it to an anthem-with-flash is the rare case where the mechanic guards an aggressive play rather than a controlling one, protecting the combat trick on the stack even if it cannot protect the creature forever.