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