[RPG] Will the illusion of a creature, of a given type, trigger a spell contingent upon the creature type

dnd-5eillusionspells

While exploring the answer to this question the question arose about whether or not the illusion of a creature type would have sufficed for magic requiring a given creature type to work.

Is an illusion of a dragon (or any other creature type) an acceptable stimulus to trigger a spell that specifies a dragon (or any other creature type), or does the actual creature of that type have to be present? As examples I'll use the spells Magic Mouth and Alarm, although there are other spells that require triggers.

All citations are from the SRD V_5.1

  1. I instruct the magic mouth to sing "See You In September" whenever an
    aberration gets within 30' of it. My clever gnome comrade sends
    the illusion of a gibbering mouther toward the magic mouth in order
    to play a prank on me. Does the illusion trigger the magic mouth, or do we need a real gibbering mouther to trigger that spell?

    Magic Mouth
    The triggering circumstance can be as general or as detailed as you like, though it must be based on visual or audible conditions that occur within 30 feet of the object. For example, you could instruct the mouth to speak when any creature moves within 30 feet of the object or when a silver bell rings within 30 feet of it. (p. 161)

  2. I set the trigger for an alarm to alert me if a dragon enters the alert area. An illusionist who has been our party's nemesis moves an illusion of a wyvern (using the spell major image) into that area while doing reconnaissance before sending his minions after us.

    Alarm

    Until the spell ends, an alarm alerts you whenever a Tiny or larger creature touches or enters the warded area. When you cast the spell, you can designate creatures that won’t set off the alarm. You also choose whether the alarm is mental or audible. (p. 114)

Bottom Line

Does the illusion suffice as criteria for either of the spells to trigger, or does a spell with such a trigger need the real thing?

Put in other words, can spells be fooled by illusions? (Thank you @Mindwin).

Notes:

  1. Creature Types are (p. 254, 255): Aberrations, Beasts, Celestials, Constructs, Dragons, Elementals, Fey, Fiends, Giants, Humanoids, Monstrosities, Oozes, Plants, Undead.
  2. Gibbering Mouther (p. 314): Medium aberration, neutral
  3. Wyvern (p. 356): Large dragon, unaligned

Best Answer

Magic Mouth should trigger, but not Alarm

As always: many things are ultimately up to the DM, but the term "visual or audible conditions" in Magic Mouth implies only a visual/audible 'sensor'. Illusions should be able to fool visual/audible sensors, while a silent/invisible aberration should not trigger the Magic Mouth (by RAW it did not meet the visual/audible conditions). Giving the Magic Mouth spell greater detection abilities than this would seem unwarranted, and could lead to unexpected uses or abuses.

Conversely, the sensor for the Alarm spell somehow detects when a "Tiny or larger creature touches or enters the warded area" without mentioning sight/hearing. I'd use strict RAW here: an illusion wouldn't fulfill the triggering condition, but an invisible creature would.

For other spells - I'd similarly base the trigger detection abilities off their description. Examples:

  • Detect Magic: "see a faint aura around any visible creature or object" implies that you wouldn't see it around an illusion (per Jeremy Crawford), but the trigger detection should still be fooled by Invisibility (an illusion spell) due to the "visible" clause in the above statement.
  • Glyph of Warding can activate "under certain circumstances or according to physical characteristics (such as height or weight), creature kind (for example, the ward could be set to affect aberrations or drow), or alignment". So we know that it can detect all those things. But having it detect say, 'someone that lied in the last 5 minutes' (to serve as a lie detection spell) wouldn't seem warranted.
Related Topic