Keith said:
Mirror Image broken? How many years of bad luck is that?
1d4+1/3 levels of caster (maximum 7 years)?
Casters select their targets when casting the spell and then when the spell has been cast it affects the target(s) if it is able to. The descriptive text for
Magic Missile states that "Inanimate objects cannot be damaged by the spell". You can choose to target something that is not a creature but the missile(s) would impact with no effect as it was not a valid target. The same would go for something like
Hold Person. You'd choose a target that happens to be invalid and then once you have cast the spell you find out that it didn't work.
The
Mirror Image descriptive text states that any successful attack against a figment destroys it. For a
Magic Missile to successfully attack one of the figments it would have to be able to affect figments, which it can't. The flipside comes from the section of
Mirror Image which states "While moving, the character can merge with and split off from figments so that enemies who have learned which image is real are again confounded". This means that when someone does get lucky and hits the real person that person can be in for some serious grief until they are able to merge/shift/split with the images to disguise their position again on their next turn.
For example, you've got an archer delaying to wait for an allied wizard to shoot a volley of
Magic Missiles at an enemy wizard who has
Mirror Image running. They get lucky and the archer sees one "image" of the enemy wizard take an impact from one of the missiles while the other missiles merely pass through their targets with no effect. He then acts and rapid fires at that "image".
If you allow
Magic Missiles to destroy the
Mirror Image figments in addition to potentially identifying the real caster (by noting the impacted image which doesn't disappear) I think it takes away from the usefulness of
Mirror Image and simultaneously adds to the usefulness of
Magic Missile.
Mirror Image isn't so powerful that it needs more ways to take it down, while
Magic Missile is already doing pretty well for a level one spell.
Lastly, I would not go with the ruling that destroying an image satisfies the requirements for a Cleave attack. A fighter in melee with a caster running
Mirror Image is already likely to chop through the images in a few rounds. A fourth level fighter with Great Cleave already has some decent odds of either destroying all the images in one round or destroying images until he hits the real caster. So a spell which should have bought the wizard at least a couple of rounds of breathing space has just evaporated in a frenzy of sword swinging. At fourth level the wizard might be able to cast
Mirror Image once during the fight, while the fighter can go on a Great Cleaving parade every round. By sixth level the fighter can open up a can of whirlwind on the wizard and his images, but by then the wizard has learned some new tricks. So the arms race goes.