You’ve got this game in the bag. You’ve got your Polukranos on the battlefield, and your opponent’s only creature is a Deadly Recluse, which is tapped because it attacked last turn. All you have to do is monstrify your Polukranos to increase its power slightly, then swing for the win. So you activate your Polukranos with X equal to 2, you prepare to swing, but your opponent stops you and says your Polukranos is dead. So what happened there?
You might read Polukranos and see that its says “any number”, and think that you can choose 0 targets, and you would be mostly correct. However, if you’re dividing something like damage, and there is a legal target for the ability, you must choose that target. That means if your opponent controls a creature that can be targeted, and you monstrify Polukranos with X equal to one or more, you have to choose that creature as a target for the ability. If your opponent does not control a creature, you can still monstrify Polukranos and it will get the counters, but then the ability that triggers when Polukranos becomes monstrous will be removed from the stack because no legal targets can be chosen, so Polukranos will get the counters and not have to fight anything.
So going back to the original situation, when you monstrify Polukranos, you have to choose up to two targets (since you have two damage to divide). Since the opponent controls a creature (the Recluse), Polukranos must target it with the ability. Because of deathtouch, Polukranos will die along with the Recluse, and you just spent a lot of mana to trade your Polukranos for a spider.
Today’s Rules Tip written by Nathan Long