A player in my group had grappled a caster. The caster then cast mirror image.
The player says since he grappled the creature he's unaffected by the illusion: he can feel the one that he had grabbed. Since that's the case the duplicates can't stop the attack, since the grappling character knows which image is real.
My question is: do the illusions have their usual chance to stop the attack or does mirror image have no effect on him since his touch can know he's holding the caster?
To quote a section of Mirror Image:
A creature is unaffected by this spell if ... it relies on senses other than sight
In your situation the "grapple" to me (if done prior to the casting of Mirror Image) would not affect the grappler (since he can rely on touch instead of sight).
The grappling rule say on page 195 of the PHB
When you want to grab a creature or wrestle with it, you can use the Attack action to make a special melee attack, a grapple...If you succeed, you subject the target to the grappled condition (see appendix A). The condition specifies the things that end it, and you can release the target whenever you like (no action required).
The grappled condition specifies it ends on
• The condition ends if the grappler is incapacitated (see the condition).
• The condition also ends if an effect removes the grappled creature from the reach of the grappler or grappling effect, such as when a creature is hurled away by the thunderwave spell.
(page 290 PHB)
and looking at the mirror image spell
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 o f your duplicates.
(page 260 PHB)
This leads me to believe since the PC, in the grapple, made the attack before the spell was cast, then that PC would be unaffected by it. This is because the only time I saw that the spell triggers is during an attack.
The question is inferring to the ability of the grappler to make an attack on a grappled subject who has casted Mirror Image after having been grappled.
The full description sighted at the end of Mirror Image reads as:
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.
(page 260 PHB)
Once we have the full description we can see that the spell is referencing a special ability and a catogory that D&D 5e refers to as senses. Further research will lead towards both the Dungeon Masters Guide (page 279 DMG) and the Monster Manual.
STEP 19. SENSES
A monster might have one or more of the following special senses, which are described in the Monster Manual: blindsight, darkvision, tremorsense, and truesight. Whether the monster has special senses or not has no bearing on its challenge rating.
On pages 8-9 of the MM in another section titled Senses it goes on describe the senses in detail. It is of note that DMG is refering to the creation of monsters and the MM to describing monster abilities, but in D&D monsters infer to the creation of enemy NPCs that can also include humans, dwarves, elves and such. Therefore these are the senses that the spell is pointing at since it includes actual examples of what it refers to as senses. If the grappler has none of these "senses" then it should be understood that the spell comes into effect, even after the caster has been grappled.
A popular argument also seems to point at "touch" as being a sense, and for the sake of argument let's run through it. Page 204 of the PHB describes illusion spells as follows:
Illusion spells deceive the senses or minds of others. They cause people to see things that are not there, to miss things that are there, to hear phantom noises, or to remember things that never happened. Some illusions create phantom images that any creature can see, but the most insidious illusions plant an image directly in the mind of a creature.
This is very important since this strengthens the case about special "senses". Touch is interpreted though the brain aka the mind. Therefore if you do not posses blindsight for example, you cannot perceive what you touch as real.
I still would allow the grapple to continue, there is no escaping that. But once the grappler takes an attack action he is subject to the conditions of Mirror Image.
If the DM still doesn't see it that way, and house rules that touch effects the casting of the spell, minimum I would recommend the grappler at least spend as action doing a Wisdom Perception check. Even a cantrip like Minor Illusion has a condition like that for physical interaction with an illusion.
If a creature uses its action to examine the sound or image, the creature can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC. If a creature discerns the illusion for what it is, the illusion becomes faint to the creature.
(page 260 PHB)
