Tabletop
Ack! I'm so sorry! I thought I posted already. The images only disappear on a direct attack, according to the mirror image2nd-level illusion
Casting Time: 1 action
Range: Self
Components: V, S
Duration: 1 minute
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.
A creature is unaffected by this spell if it can't see, if it relies on senses other than sight, such as blindsight, or if it can perceive illusions as false, as with truesight. spell. It's very annoying really because of Mourne's ability to "leap" into another image. The line that matters is, "It ignores all other damage and effects. " Of course, normally, mirror image doesn't take up any extra squares, so it's a bit of an unusual version.
The fireball would damage Mourne; mirror image doesn't protect him, but neither do you look images because it's immune to other damage effects.
Okay, on to Troben's turn! He will try to attack Mourne, but must make the Wisdom saving throw first (DC 18): Saved!!
Troben uses a bonus action to summon an echo next to the Mourne's and attacks him! He's got plenty of "movement" to go between images. Or if you rule that Eithne's blast destroyed them, well...then it's easy to pick!
- Attack #1 - hits AC 15. That would hit an image though, but probably not Mourne.
- Attack #2 - hits AC 25 doing 18 slashing and 4 fire damage.
If you could give us Mourne's AC, that would be helpful for future rounds.