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ILLUSIONS 101
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LIMITS TO USING ILLUSIONS

* Most visual illusions are useless to fool creatures with blindsight or truesight, but non-living creatures (undead and constructs) are not automatically immune to such trickery unless noted in the creature's description.

* Most illusion spells can be foiled by a successful Investigation (Intelligence) check. "An ability check is used instead of a save when an illusion is cast at the environment, rather than at a creature." (Quote from 11/20/2014 tweet from D&D5e editor @JeremyECrawford.)

* As a rule of thumb, a lower level illusion spell cannot replicate the specific effects granted by a more specific, higher level spell. For example, Silent Image (1st level) cannot cover an individual in shimmering waves and gain the defensive bonus described in the Blur illusion spell (2nd level). 

* Light and darkness are not "objects," thus the Minor Illusion cantrips cannot duplicate the obscuring effects of a fog cloud, nor Light or Darkness spells. (Confirmed in 9/17/2015 tweet from D&D5e editor @JeremyECrawford.)

* Objects created by the Minor Illusion cantrip cannot be made to move. (Confirmed in 8/19/2015 tweet from D&D5e editor @JeremyECrawford.) Silent Image (2nd level) and Major Image (3rd level) specifically allow moving objects or creatures within the illusion's area of effect, though it requires using an action to trigger such movements.

* Silent Image and Major Image illusions "covering" something cannot make real things invisible, nor render an area lightly or heavily obscured. Only the specific Invisibility spell (2nd level), Greater Invisibility spell (4th level) or Mislead spell (5th level) can make something disappear from sight.

COUNTER-POINT: That ruling seems to contradict the consequences of an affective Investigation check, which if successful the illusion is still seen but very transparent. If that Investigation check fails and the illusion appears solid -- so why can't "real" objects hide behind it?

SUGGESTED HOUSE RULE: Creatures "inside" an illusion disrupt its effectiveness, like hands waving through smoke. It is up to DM to allow creatures the Hide action while "inside" or "behind" an illusion's area of effect. (Maybe grant advantage on the Hide check if illusion fully covers the character?)

CANTRIPS
* Minor Illusion: Makes a single sound (talking allowed) or non-moving object (not a creature) in 5-foot cube area. Touching spoils illusion.

1st LEVEL SPELLS
* Disguise Self: Specialized to one individual. Includes carried items.
* Illusory Script: Designated creature can read, others see jibberish. Lasts 10 days. 
* Silent Image: Moving images in 15-foot cube area. Touching spoils illusion.

2nd LEVEL SPELLS
* Mirror Image: Specialized interactive duplicates.
* Phantasmal Force: Cannot be perceived by others, only appears in subject's mind.

3rd LEVEL SPELLS
* Major Image: Moving images in 20-foot cube area, including appropriate sites, sounds, smells and non-damaging temperatures. Can replicate visuals of creatures. Touching spoils illusion.

4th LEVEL SPELLS
* Hallucinatory Terrain: Natural terrain in 150-foot cube area, including sounds and smells. Building and creatures still appear as normal, unchanged by the illusion magic.
* Phantasmal Killer: Cannot be perceived by others, only appears in subject's mind.

5th LEVEL SPELLS
* Creation: Makes 5-foot cube of material.
* Dream: Cannot be perceived by others, only appears in subject's mind.
* Mislead: Caster invisible, replaced by moving duplicate.
* Seeming: Essentially mass Disguise Self.

6th LEVEL SPELLS
* Programmed Illusion: As Major Image in 30-foot cube area, but only starts when a specific trigger happens. Last until dispelled, "loops" each time trigger happens.

7th LEVEL SPELLS
* Mirage Arcane: Essentially Hallucinatory Terrain over 1-mile square, but also affects buildings (but not creatures).
* Project Image: Duplicate of caster looks, sounds, smells, acts normal for 24 hours. Touching spoils illusion.
* Simulacrum: False duplicate create lasts until dispelled.

9th LEVEL SPELLS
* Weird: Cannot be perceived by others, only appears in subjects' minds.