During a discussion on how mirror image interacts with area of effect spells, and particularly with magic missile, the question arose about how faerie fire interacts with mirror image. This is related to the general question of how mirror image interacts with spells that don't have an attack roll but is more specific.
The faerie fire spell description states:
Each object in a 20-foot cube within range is outlined in blue, green, or violet light (your choice). Any creature in the area when the spell is cast is also outlined in light if it fails a Dexterity saving throw. For the duration, objects and affected creatures shed dim light in a 10-foot radius.
Any attack roll against an affected creature or object has advantage if the attacker can see it, and the affected creature or object can't benefit from being invisible.
The description of mirror image says:
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.
If you have three duplicates, you must roll a 6 or higher to change the attack's target to a duplicate. With two duplicates, you must roll an 8 or higher. With one duplicate, you must roll an 11 or higher.
A duplicate's AC equals 10 + your Dexterity modifier. 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.
Does faerie fire only light up the creature who cast mirror image, or do all of the illusory duplicates also get outlined/lit up by faerie fire?
Our guess was that faerie fire lights up all of the images, but since faerie fire is working like an AoE spell, and AoE spells aren't fooled by the images (fireball, etc), perhaps we misunderstood how these two spells interact.