When you cast your next instant or sorcery spell this turn, copy that spell. You may choose new targets for the copy.
Raid — If you attacked with a creature this turn, when you cast your next instant or sorcery spell this turn, copy that spell an additional time. You may choose new targets for the copy.
9/20/2014 Howl of the Horde creates a delayed triggered ability that triggers when you cast your next instant or sorcery spell that turn. The raid ability creates a second, identical delayed triggered ability. Each of these will create a single copy of the next instant or sorcery spell you cast during that turn. If you don’t cast another instant or sorcery spell during that turn, both delayed triggered abilities will cease to exist.
9/20/2014 The delayed triggered abilities will copy any instant or sorcery spell, not just one with targets.
9/20/2014 When either ability resolves, it creates a copy of the instant or sorcery spell. You control each copy. Each copy is created on the stack, so it’s not “cast.” Abilities that trigger when a player casts a spell won’t trigger. The copy or copies will then resolve like normal spells, after players get a chance to cast spells and activate abilities.
9/20/2014 Each copy will have the same targets as the spell it’s copying unless you choose new ones. You may change any number of the targets, including all of them or none of them. If, for one of the targets, you can’t choose a new legal target, then it remains unchanged (even if the current target is illegal). You can choose different targets for each copy.
9/20/2014 If the spell being copied is modal (that is, it says “Choose one —” or the like), each copy will have the same mode. You can’t choose a different one.
9/20/2014 If the spell being copied has an X whose value was determined as it was cast (like Crater’s Claws has), each copy will have the same value of X.
1/19/2018 Raid abilities care only that you attacked with a creature. It doesn’t matter how many creatures you attacked with, or which opponent or planeswalker controlled by an opponent those creatures attacked.
1/19/2018 Raid abilities evaluate the entire turn to see if you attacked with a creature. That creature doesn’t have to still be on the battlefield. Similarly, the player or planeswalker it attacked doesn’t have to still be in the game or on the battlefield, respectively.