So your opponent has a 5/5, and just cast the Onslaught to deal five damage among your creatures, and you’re hoping to use Murder to prevent that from happening. Unfortunately, your removal spell isn’t going to protect you from this Onslaught. Monstrous Onslaught looks at the highest power among your creatures when you cast it, and using that number, tells you how much damage you get to assign with the Onslaught (including what creatures will be assigned damage and how much damage is assigned). You’re free to cast Murder on their 5/5 in response to the Onslaught, but that won’t really help your creatures: they’ll still be dealt damage as assigned when the Onslaught was cast.
But keep in mind that since the damage assignment happens as part of the Onslaught being cast, you can respond to the spell knowing how much damage will be assigned and to where. You could cast something like Highspire Infusion on one of the targets to boost it enough to save it, and your opponent won’t be able to assign more damage to that creature, since the damage assignment has already been locked in. And finally, while they can target multiple creatures with the Onslaught, they still have to assign at least one damage to each creature. While there may not be anything in Kaladesh block that cares, if you wanted to take advantage of something like heroic from Theros block, you’d have to assign at least one damage to each creature; you can’t just target creatures just to target them (and accordingly, you can’t choose more targets than you have damage to distribute).
Lastly, don’t forget that a spell is only countered by the game rules if ALL its targets have become illegal before it resolves. If the Onslaught targets a 3/3 and a 2/2, distributing 3 and 2 damage respectively, but the 3/3 gets blinked by Acrobatic Maneuver, the 2/2 still takes the 2 damage that was distributed to it even though the 3/3 is no longer a legal target.
That’s it for this week. Enjoy your weekend, and we’ll see you all next week!
Today’s Rules Tip written by Nathan Long