Mizzium Mortars deals 4 damage to target creature you don't control.
Overload(You may cast this spell for its overload cost. If you do, change its text by replacing all instances of "target" with "each.")
4/15/2013 If you don’t pay the overload cost of a spell, that spell will have a single target. If you pay the overload cost, the spell won’t have any targets.
4/15/2013 Because a spell with overload doesn’t target when its overload cost is paid, it may affect permanents with hexproof or with protection from the appropriate color.
4/15/2013 Note that if the spell with overload is dealing damage, protection from that spell’s color will still prevent that damage.
4/15/2013 Overload doesn’t change when you can cast the spell.
4/15/2013 Casting a spell with overload doesn’t change that spell’s mana cost. You just pay the overload cost instead.
4/15/2013 Effects that cause you to pay more or less for a spell will cause you to pay that much more or less while casting it for its overload cost, too.
4/15/2013 If you are instructed to cast a spell with overload “without paying its mana cost,” you can’t choose to pay its overload cost instead.