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Mizzium Mortars deals 4 damage to target creature you don't control.
Overload {3}{R}{R}{R} (You may cast this spell for its overload cost. If you do, change "target" in its text to "each.")
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.
Casting a spell with overload doesn't change that spell's mana cost. You just pay the overload cost instead.
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.
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.
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.
Note that if the spell with overload is dealing damage, protection from that spell's color will still prevent that damage.
Overload doesn't change when you can cast the spell.
