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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.

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  • \$\begingroup\$ Would it be pertinent to answer the question of how the advantage applies here or should we create a new question? \$\endgroup\$ – Premier Bromanov Oct 5 '18 at 15:58
  • \$\begingroup\$ @PremierBromanov I think that might be worth its own question, making sure to link it to this one since it is related. \$\endgroup\$ – KorvinStarmast Oct 5 '18 at 16:28
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Only the creature gets hit with faerie fire, but all the images light up

Mirror image says:

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.

Note that the spell says that its main effect is to make it impossible to tell the original from the images. This could not be true if the images did not change appearance with the real creature.

If the creature takes off their shirt, the images take off shirt. If a creature gets a vat of paint dumped on them, the images also become painted. If the creature gets hit with a spell that changes their appearance, the images also change to match. If the spell would not do this then simply hitting a creature enough to rip a shirt or put a blood spot on them would be enough to negate the entire spell's effect.

So, the creature is the only one to be targeted by the faerie fire, but, because the effects of mirror image specifically make it so that the images are impossible to tell apart from the real thing, the images also appear to be affected as well.

Mechanically, this illumation will have no effect on anything about the duplicates except their appearance because:

A duplicate can be destroyed only by an attack that hits it. It ignores all other damage and effects.

In other words, creatures only get advantage on attacks against the real creature; the images disregard faerie fire completely (no advantage against them).

Rules as intended agrees

Jeremy Crawford, official rules designer for 5e, addresses a related question here:

Q: if you have mirror images out and then cast invisibility on yourself, do the mirror images also become invisible?

Jeremy Crawford: Mirror image creates illusory duplicates of you that imitate your appearance. If your appearance goes away—you become invisible, for example—then the duplicates imitate that too.

Since JC rules that even magical invisibility will be imitated by the duplicates, it seems obvious that faerie fire would fit that ruling perfectly as well.

tl;dr The creature is the only one affected by faerie fire, but all the images do still mimic the appearance of being under the effects of it.

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    \$\begingroup\$ Would this mean that the attacks you need to resolve against the mirror images would be at advantage or not? \$\endgroup\$ – NautArch Oct 4 '18 at 18:53
  • \$\begingroup\$ @NautArch: Read the last line above RAI. No, images do not get advantage against them. Should I make it clearer? \$\endgroup\$ – Rubiksmoose Oct 4 '18 at 18:53
  • \$\begingroup\$ Given that it's a major mechanical affect (or in this case not one), I think it'd be helpful to make it clearer. \$\endgroup\$ – NautArch Oct 4 '18 at 19:00
  • \$\begingroup\$ That would also mean that you actually lose the benefit of Mirror Image while you were invisible following that Tweet's logic. Since they don't also mimic sound a blind attacker could target by sound instead, toss up on which is more difficult though I guess. \$\endgroup\$ – Slagmoth Oct 4 '18 at 19:11
  • \$\begingroup\$ Faerie fire grants advantage when attacking anything it illuminates. What's the reasoning behind that not applying to the mirror images? \$\endgroup\$ – Mark Wells Oct 5 '18 at 1:13
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All images are illuminated, but there is no benefit.

Since these spells cause two distinct and specific effects, we can't rely on the rule of thumb that specific beats general. Instead, we need to find a ruling that harmonizes both specific effects. I believe there is only one sensible and consistent ruling.

Note the following from the text of the mirror image spell (emphasis mine):

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.

The only way for faerie fire to affect its target in such a way that it's still impossible to track which image is real is for all the target's mirrored images to appear to be illuminated. Any other ruling would make it literally possible to tell the images apart, which mirror image expressly forbids.

Then note the following from the text of the fairie fire spell (emphasis mine):

Any attack roll against an affected creature or object has advantage...

Despite the appearance that the mirror images are illuminated, there is no mechanical impact, because the images are not creatures or objects (as they are not indicated to be either).

To summarize: the target and all images appear illuminated, but there is no practical mechanical benefit except against the target creature itself.

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I agree that the figures light up, even if they are not affected by the spell itself.

But I would also argue that you get the advantage against them.

The main argument against is that the immages are not affected by the spell, but since the feary fire advantage is a purely visual effect that makes it easier to hit (which is why you get no advantage if you can't see it) I would say they would copy that any way same as with invisibility. You either include all the effects because of vision (including the advantage) or none (in which case you can see which is real). If all the weak points are highlighted on you, they are also highlighted on the images, thus the images would also be easier to hit.

If those are not shown, then it would be easy to distinguish the real thing since there are visual differences which the mirror image should not allow.

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I read it as the mirror images also being affected because, by RAW, spells do what they say they do, and only outlining the real person would either:

  1. Only outline the benefactor of Mirror Image, giving advantage to hit him, but with the benefits of Mirror Image staying, which is silly (I can tell which one is the real guy, but I hit his duplicate instead. Oops.), or

  2. Faerie fire becomes a hard counter to Mirror Image. Seeing as the spell only calls out Invisibility, I don't think that it's RAW or RAI for Mirror Image to become useless when someone blasts them with Faerie Fire.

Personally, I would rule it as the Faerie Fire only affecting the target, but the fire also being mirrored, making it appear to target the duplicates as well.

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    \$\begingroup\$ I don't think you're necessarily wrong with your final decision - but you are missing any support beyond your opinion. \$\endgroup\$ – NautArch Oct 4 '18 at 18:22

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