Cone of Flame has three targets: a target creature or player, another target creature or player, and a third target creature or player. You have to choose three different targets for it. You can’t target the same creature or player multiples times with it (sorry, the Cone is good, but it’s not good enough to take down one of the mythic Souls by itself). And if there aren’t three legal targets for it, you’re not going to be able to cast it at all. Luckily, if you really just want to be rid of your opponent’s Scrapyard Mongrel, but there’s no other creatures on the battlefield, there is a way to kill it.
The Mongrel is a legal target, but in most cases, there’s two other legal targets as well: you and your opponent. There’s nothing preventing you from targeting yourself if you really need to cast the Cone (just try to make sure you choose yourself as the target for one damage, unless you have other plans.
And one final note: how much damage each creature or player is assigned is chosen when you announce the spell, not when it resolves. For instance, if you target three of your opponent’s 1/1s with the Cone, and they respond by casting Gather Courage on the one that was assigned one damage, you can’t change your mind and say that you want to deal 3 damage to that creature instead – that choice was locked in when you announced the spell, and it can’t be changed later on.
Today’s Rules Tip written by Nathan Long