Here’s a list of guidelines for handling illusions:
From the Player’s Handbook
- 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).
- Illusionary monsters attack using the wizard’s attack values. This would be a subtle clue that the monsters are fake.
- 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).
- 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.
- For illusion spells that require caster concentration to maintain, the caster may only maintain a number of spells equal to his level. The spells that can be maintained simultaneously can be of any level. Note, the caster must still cast each spell sequentially, so it will take time to stitch together a complex illusion utilizing many spells and some spells with duration may have to be renewed. Even spells that do not explicitly require concentration (like audible glamer) do require it if the caster wants the illusion to be reactive. For example, audible glamer cast to simulate the sound of a waterfall can be maintained without concentration, but audible glamer cast to supplement a phantasmal force illusion where a group of illusory fighters respond verbally to questions asked of them DOES require concentration.
- Once an illusion is cast, the caster can make the elements react to the environment for the duration of the spell. Introducing new elements, however, requires a new casting.