We are planning an attack on our nemesis. Intelligence efforts indicate that the enemy can use mirror image. I have green-flame blade. As we thought through our tactics the question came up:
Would GFB, on a hit, remove more than one of the mirror images? We think not, but we think that it will by default do damage to mirror image's caster.
What we hope happens: If I hit the image, the additional green flame hits the caster
[...] make a melee attack with it against one creature within 5 feet of you. On a hit, the target suffers the weapon attack's normal effects, and you can cause green fire to leap from the target to a different creature of your choice that you can see within 5 feet of it. The second creature takes fire damage equal to your spellcasting ability modifier.
The only creature within five feet (assuming that I am one-on-one with our nemesis) is the one who cast mirror image. If I hit them, we think that the green flame does not spread to one of the images (nor damage them) but if I end up hitting one of the three images, the only creature available for the flame to hit is the caster himself. So it does so, by default. mirror image states:
Three illusory duplicates of yourself appear in your space. Until the spell ends, the duplicates move with you and mimic your actions, shifting position so it's impossible to track which image is real. You can use your action to dismiss the illusory duplicates.
Each time a creature targets you with an attack during the spell's duration, roll a d20 to determine whether the attack instead targets one of your duplicates.
The additional flame damage, we are pretty sure, won't hurt any of the duplicates.
If an attack hits a duplicate, the duplicate is destroyed. A duplicate can be destroyed only by an attack that hits it. It ignores all other damage and effects. The spell ends when all three duplicates are destroyed.
Example of how we think it works: sixth level caster, cha of 18, attacks with GFB, hits. Duplicate destroyed, 1d8+4 green flame damages adjacent creature (the caster of mirror image).
If my GFB hits a mirror image duplicate with the attack, does the additional damage automatically damage the creature who cast mirror image? (As long as no ally of that creature is within 5')
Does it maybe hit (to no effect) one of the other duplicates?
(The related consideration is how does one discern what it damages: the caster or the duplicate?)