Adjudicating Illusions

All illusions are cases of DM adjudication; each depends upon the exact
situational factors deemed significant by the DM. All of the following points are
only subsidiary guidelines to help the DM maintain consistency.

Intrinsically Deadly Illusions: “Instant kill” illusions that are automatically fatal regardless of level, Hit
Dice, or saving throws: collapsing ceilings, inescapable lava pits, etc. The
absolute maximum effect of these is to force a system shock check. Surviving
characters are not further affected by that illusion.

Spell Effects: Illusions that duplicate spell effects are keyed to the caster’s level (for
example, a 10th-level illusionist casting a fireball can create a convincing
10-die fireball). Exceeding this limit creates a fatal flaw in the illusion that
negates its effect.

Monster Special Attacks: Before the caster can effectively duplicate a monster’s special attack, the
wizard must have undergone it (a wizard cannot conjure up the twinkle in a
medusa’s eye correctly without actually experiencing it–i.e., having been turned to
stone by one).

Option: Illusionary monsters attack using the wizard’s attack values. This would be a
subtle clue that the monsters are fake.

Option: Extend the spell level control to monsters–the caster can create monsters
only if the total monster Hit Dice are equal to or less than the caster’s level
(an 8th-level caster could convincingly do one hill giant, two ogres, or four
2nd-level fighters).

Illusion spells require a higher degree of DM-player interaction than other
wizard spells. The timing and staging of such spells by the caster are extremely
important. Effects that appear out of nowhere are not believed unless the
caster takes this into account. On the other hand, an illusionary fireball cast
after a wizard has cast a real one could have devastating effects.

The caster must maintain a show of realism at all times when conducting an
illusion (if a squad of low-level fighters is created, the caster dictates their
hits, misses, damage inflicted, apparent wounds, and so forth; the DM decides
whether the bounds of believability have been exceeded).

NPC illusions require careful preparation by the DM, including clues to their
nature.

Intelligence is the best defense against illusions. Low and nonintelligent
creatures are more vulnerable to illusions, unless the illusion is completely
outside their experience or the illusion touches on an area of the creatures’
particular competence. Undead are generally immune to illusions, but they are
vulnerable to quasi-real effects, most of which start to appear in the 4th-level
spell list.

Illusions usually cease to affect a character if they are actively
disbelieved. Disbelief must be stated by the player, based on clues provided by the DM.
Players stating disbelief must give a reason for disbelief based on sensory
information available to the character. Failure to give such a reason results in
failure to disbelieve. The DM can impose additional requirements or delays in
recognizing illusions (such as Intelligence checks) as needed, such as when one
player is obviously parroting a discovery made by another. Disbelief automatically
forfeits a saving throw if the effect is real.

For NPCs, a saving throw, Intelligence check, or DM adjudication can be used
to determine disbelief (whichever the DM deems appropriate).

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