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An “attacking creature” is one that has been declared as an attacker or put onto the battlefield attacking during a turn’s combat. Unless that creature leaves combat, it continues to be an attacking creature through the end of combat step, even if the player it was attacking has left the game or the planeswalker it was attacking has left combat.

If you cast Wing Shards after combat damage has been dealt, only creatures that survived combat can be sacrificed.

Storm counts spells cast before the spell with storm was cast. Spells cast after the spell with storm was cast but before the storm ability resolves aren’t counted.

The copies storm creates are created on the stack, so they’re not cast. Abilities that trigger when a player casts a spell (such as storm) won’t trigger.

The storm ability and the copies it creates all resolve before the spell with storm. They resolve even if the spell with storm is countered.

Wing Shards doesn’t target the creature to be sacrificed. The target player chooses the creature as it resolves.