According to the Monster Manual, a variant Vampire spellcaster can cast the Mirror Image spell. If vampires aren't reflected in mirrors and other reflective surfaces would a mirror reveal reflections of just the magical mirror images or would it reflect nothing?
[RPG] Would a mirror image of a vampire be reflected in an actual mirror
dnd-5emonstersspells
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No, mirror images aren't enemy creatures
Sneak attack requires "another enemy". Mirror images aren't enemies different from you; they're in your space, and they "move with you", so they don't present additional threats.
In general, there's no strict definition of "enemy" provided in the PHB, but it's often used interchangeably with "hostile creature". So, as you continue to find ways to enable Sneak Attack for your Arcane Trickster, consider the Sneak Attack rule to be:
You don’t need advantage on the attack roll if another creature hostile to the target is within 5 feet of it, that creature isn’t incapacitated, and you don’t have disadvantage on the attack roll.
Mirror images aren't hostile creatures, so this won't work. RAI for allowing sneak attacks without advantage when an enemy is adjacent to the target is clearly that sneak attacks are facilitated by the rogue supplementing other party members in combat, so low-level illusions don't cut it. Note that you don't have to be in melee; you can stand back and snipe with missile weapons while your ally gets in close.
(As a general suggestion, I would work on using your Arcane Trickster's spells to buff yourself/teammates and make ranged sneak attacks rather than trying to make melee sneak attacks work without teammates to help you).
Mirror image only affects attacks, and blight and magic missile aren't attacks.
Mirror Image (PHB p.260):
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.
Attacks are only things that require an attack roll (weapon attacks and spell attacks). See P. 194 of the PHB:
If there’s ever any question whether something you’re doing counts as an attack, the rule is simple: if you’re making an attack roll, you’re making an attack.
("Attack" has a specific rules meaning in 5e that isn't the same as its ordinary English meaning; actions and spells that might be described as attacks in ordinary language aren't necessarily attacks as far as the 5e rules are concerned.)
If a spell doesn't say that the caster makes a spell attack, then mirror image doesn't interact with it. This is clear from the complete spell description above: the spell effects only occur if the caster is targeted with an attack, and its effects consist only of changing the target of the attack to one of the duplicates. The only way to determine if a duplicate is hit is by resolving the attack against the duplicate's AC. There is no provision for mirror image interacting with spells that (for example) require the target to make a saving throw.
It might be argued that the first paragraph, particularly the phrase it’s impossible to track which image is real, establishes that mirror image makes reliably targeting the caster with any effect impossible. There are other spells that interfere with the targeting of non-attack spells, and they spell out those effects specifically:
Nondetection (PHB p.263):
The target can’t be targeted by any divination magic ...
Antimagic field (PHB p.213):
Spells and other magical effects, such as magic missile and charm person, that target a creature or an object in the sphere have no effect on that target.
If mirror image affected the targeting of non-attack spells, we would expect there to be a specific clause in the description that spells that interaction out. For example, something beginning with "When a creature targets you with a non-attack spell ... ". But there is no such wording; the effects described apply only to attacks.
It might also be argued that mirror image clearly creates illusions which are valid targets for spells, so a hostile spellcaster might target one of these illusions by accident.
However, mirror image doesn't create illusions; it creates illusory duplicates, and it specifically spells out all of their effects. Spells that create illusions (such as major image or minor illusion) have different effects (and they don't specify what happens if they are targeted by a spell, attack or otherwise).
So, to consider the two spells in the question:
Blight (PHB p.219):
Necromantic energy washes over a creature of your choice that you can see within range, draining moisture and vitality from it. The target must make a Constitution saving throw.
Magic missile (PHB p.257):
You create three glowing darts of magical force. Each dart hits a creature of your choice that you can see within range.
(emphases mine).
Magic missile just hits the actual creature being affected by mirror image, and doesn't affect the images at all. It's not an attack, because no attack roll is made; it just does damage. Blight likewise ignores the images and affects the target, as it also is not an attack.
Best Answer
I sense some confusion in some of the other answers. I think I understand what you are asking, though.
A vampire can certainly cast mirror image and create illusory duplicates of itself. This is born out by the answer to this question, and solidified by the fact that the spell mirror image is in the Vampire variant statblock you quoted.
It is also stated in the description of vampires in the monster manual that they cast no shadows or reflections.
The rules are silent as to whether the illusory duplicates created by the spell can themselves be seen in a mirror. The rulebook has finite space, and play groups are expected to come up with rulings, where the rules are not explicit. This is one of those things that a DM will have to adjudicate. Some DMs might rule that an illusion of a creature that cannot be seen in a mirror cannot itself be seen in a mirror. Other might rule that the illusion does not have all of the supernatural properties of the original and thus it can be seen.
However, it doesn't matter.
The spell mirror image is not built to allow people to target an individual based on some deduction of which one is real. The spell details how attacks interact with the target.
The illusions are assumed to be constantly shifting. Even if there were seemingly four vampires in the area (the same five foot square), and a mirror did indeed only reflect three of them, this would give you no ability to target the real one any better. The tightly grouped images constantly shift through the duration. If you want to hit the vampire, you will need to use the targeting limitations imposed by the spell.