This section is intended to clear up exactly how illusions are dealt with in this campaign, regarding saving throws and definitions of “interaction” with illusions.
The Pathfinder Core Rulebook states:
Illusion spells deceive the senses or minds of others. They cause people to see things that are not there, not see things that are there, hear phantom noises, or remember things that never happened.
Figment: A figment spell creates a false sensation. Those who perceive the figment perceive the same thing, not their own slightly different versions of the figment. It is not a personalized mental impression. Figments cannot make something seem to be something else. A figment that includes audible effects cannot duplicate intelligible speech unless the spell description specifically says it can. If intelligible speech is possible, it must be in a language you can speak. If you try to duplicate a language you cannot speak, the figment produces gibberish. Likewise, you cannot make a visual copy of something unless you know what it looks like (or copy another sense exactly unless you have experienced it).
Because figments and glamers are unreal, they cannot produce real effects the way that other types of illusions can. Figments and glamers cannot cause damage to objects or creatures, support weight, provide nutrition, or provide protection from the elements. Consequently, these spells are useful for confounding foes, but useless for attacking them directly.
A figment’s AC is equal to 10 + its size modifier.
Glamer: A glamer spell changes a subject’s sensory qualities, making it look, feel, taste, smell, or sound like something else, or even seem to disappear.
Pattern: Like a figment, a pattern spell creates an image that others can see, but a pattern also affects the minds of those who see it or are caught in it. All patterns are mind-affecting spells.
Phantasm: A phantasm spell creates a mental image that usually only the caster and the subject (or subjects) of the spell can perceive. This impression is totally in the minds of the subjects. It is a personalized mental impression, all in their heads and not a fake picture or something that they actually see. Third parties viewing or studying the scene don’t notice the phantasm. All phantasms are mind-affecting spells.
Shadow: A shadow spell creates something that is partially real from extradimensional energy. Such illusions can have real effects. Damage dealt by a shadow illusion is real.
Saving Throws and Illusions (Disbelief): Creatures encountering an illusion usually do not receive saving throws to recognize it as illusory until they study it carefully or interact with it in some fashion.
A successful saving throw against an illusion reveals it to be false, but a figment or phantasm remains as a translucent outline.
A failed saving throw indicates that a character fails to notice something is amiss. A character faced with proof that an illusion isn’t real needs no saving throw. If any viewer successfully disbelieves an illusion and communicates this fact to others, each such viewer gains a saving throw with a +4 bonus.
The key here is to determine what defines an interaction. In this campaign, interactions are as follows:
One must need a solid and tangible reason to disbelieve an illusion. Interaction may be either passive or active.
Passive interaction is when an illusion is created and either actively affects a character, or when a character inadvertently interacts with an area covered by an illusion. Note that according to the standard rules, figments and glamers are unreal, and cannot cause damage to objects or creatures. Illusion spells that can damage creatures have their own specific rules, listed in the individual spell’s description. There may be times, however, when a figment or glamer is used to trick or baffle a character. An illusory (figment) creature may be created to fight an enemy, but it may do no damage. Hence the illusionist must have a good view of the battle, and concentrate on the spell, making the illusory monster act and react in such a way as to make the monster’s ineffectual actions seem believable. Being attacked by an figment monster is considered a passive interaction. The DM may either have the character roll a Will saving throw vs. the illusion, or he may roll the saving throw in secret (as to not tip off the fact that the creature is an illusion). A failed save means the character believes the creature is real. It may still not damage the affected character, but the character will react in such a manner as if the monster were real (such as avoiding attacks of opportunity, moving around the perceived creature rather than through its space, etc).
Once this save is failed, the character believes the figment to be real. If he later becomes aware that the creature might be an illusion, (for a multitude of reasons), he may attempt a second passive Will saving throw. In this instance, the character trying to disbelieve must in effect let the creature hit them, trusting that the blow will pass right through him and do no damage. This means that they will not dodge or protect themselves from the blow (from that creature only, they will retain their full AC for all other creatures). If the Will save is made, and the creature is indeed an illusion, the blow passes safely through the character, who now knows it for what it is, and the figment becomes a translucent outline to the character. If this creature is indeed real, and not a figment, the blow automatically hits, for the character did nothing to avoid the blow. All other creatures who notice the blow pass harmlessly through the character may choose to make an active Will save (see below), adding +4 to the saving throw to disbelieve the creature, just as if a character who knows it to be an illusion had informed his allies that it was an illusion. If the save is failed, the character’s senses tell him that the illusion is real. At this point, only a successful active interaction will allow any further saving throws.
Another form of passive interaction is when the character is affected by an illusory spell that causes a sensory effect, or is imitating another spell which causes some sort of sensory effect. An example is an illusion of a darkness spell. As the darkness envelops the character, they are entitled to a passive Will saving throw in order to disbelieve the illusion. Failure results in the character believing the spell is real, until they are given a reason to disbelieve it in the future.
Yet another type of passive interaction is when the character interacts with what he thinks is a normal environment, but which is actually a figment or glamer, such as an illusory bridge, a section of illusory floor that disguises a pit, or an area affected by the hallucinatory terrain spell. This environment, to gain a passive Will saving throw, must be interacted with (one cannot simply stand at the edge of an illusory forest and gain a passive save to disbelieve). The bridge or floor must be walked upon, the glamered terrain must be trespassed. In effect, if the elements of the environment are illusory, and blocks or interferes with movement and/or visibility, then the characters are allowed a Perception check at the DC of the illusion spell. If they fail, they do not notice anything amiss or inconsistent with the area. They will step around figment rock outcroppings and glamer trees. If they succeed on the perception check, they notice a flaw in the environment and may then choose to actively disbelieve (see below). Note that this only applies to such spells as hallucinatory terrain or other illusions that the characters pass over or through. There is absolutely no reason for a character to disbelieve, say, a maze of illusory walls (unless of course the player states his character is trying to interact with the wall, such as touching it or drawing on it with chalk, etc). If the character does interact with it, he is allowed the passive Will saving throw. Note that others specifically watching the interacting character may also be allowed active saving throws if they notice that the chalk, for instance, passes through the wall.
In such an instance where moving over the illusion would cause deleterious consequences, such as the aforementioned illusory bridge, the character is allowed a passive Will saving throw as they step upon the bridge (or over the illusory floor). If they fail, they believe the surface to be real and continue walking. They then pass through the illusion, because it does not actually exist, suffering the subsequent falling damage (or whatever fate may befall them below). If they make their saving throw, they still pass through the bridge or floor (since it is not really there), but are allowed a Reflex save to catch themselves and avoid the fall. In this instance, their comrades are allowed to make active saving throws (see below) to see through the illusion.
Most seasoned illusionists know that their spells will be passively resisted when used to directly interact with an opponent, hence they tend to use such illusions instead to thwart, delay, or otherwise foil their enemies. Illusory walls, pits, and terrain are common. Under most circumstances, there is no reason for a character to think there is anything untoward about illusory objects, unless they have recently encountered other illusions, or they closely study the item and notice inconsistencies. The DM is not obligated to inform characters that they may be encountering static illusions. He is obligated, however, to allow for Perception checks to notice any inconsistencies, such as a babbling brook that does not make sound, or a wall of thorns that does not have the proper odor, or a dungeon wall that gives off neither cold nor heat when touched. Such Perception checks may be called for by the DM (and the DM may choose to make these PER checks in secret), or instigated by the PCs. When interacted with unawares, the characters make passive interaction saving throws as detailed above.
If a character is suspicious that the terrain they are interacting with is illusory, they must take an ACTION to disbelieve the illusion. This action needs to be a dedicated attempt to prove to themselves that the thing they are seeing is indeed not real. Such actions may include passing a hand through the illusion, stepping onto the illusion, throwing an object onto or through the illusion, or in the case with an illusory creature that has not interacted with them, to pass something through it as if it were not there, or let it try to hit them. They are then allowed a Will saving throw to disbelieve the illusion (and may even gain a bonus to the save depending on the circumstances). Note that if they fail this save, the character believes that the illusion is real, and must react appropriately. If the creature attacks them, they are allowed the passive save as detailed above. Also note that if faced with what may or may not be an illusory creature, one may not strike it for damage and attempt to disbelieve in the same action. The disbelief attempt is taken specifically to prove that the illusion does not exist. Striking it for damage proves only that the character has the idea (or fear) that the creature may indeed be real.
Another instance of active interaction would be to pierce a glamer such as disguise self. The character would need to take some action to determine if the target were indeed under the effect of an illusion. Such actions may include touching the face or clothing of the target, or attempting to study the target, listening for inconsistencies in the way their gear sounds contrasted to the way it appears.
There may be instances when, even though a character interacts with an illusion, their expectations regarding the encounter may cause them not to receive a Will saving throw to disbelieve the illusion. Such instances should be rare, and would need to be ruled on a case by case basis. An example of such an encounter would be if the character believed they were fighting an incorporeal creature. Their weapons pass harmlessly through the creature, and the creature seems not to be able to affect the character as well. It is likely that such an encounter may last an extended period of time, with the character not being allowed passive saving throws against the figment. If the character deliberately tried to actively interact with the figment in this case, to determine if it were indeed an illusion, they should be allowed to make an active Will saving throw, although certain penalties might apply depending on the circumstances.
Although only a mere cantrip, the detect magic spell usually proves useful when dealing with illusions. One must spend the required 3 rounds and make the requisite Spellcraft check (DC 15 + spell level, or 15 + 1/2 caster level for a nonspell effect) to determine if such an area or creature is an illusion. Knowing that something is an illusion is not considered interacting with it, however. The caster may inform his allies, granting them all a +4 bonus to their Will saving throws when they actively interact with the illusion. Some illusions are cast on creatures, or may be creatures themselves. Such a creature may move from the area of the detect, and the caster would be required to start over when he moves the cone of the detect spell.
If an enemy casts an illusion spell, anyone with ranks in Spellcraft may attempt to identify the spell as it is cast (DC 15 + spell level). If the spell is successfully identified, and the illusion spell is directed at the character who identified it, that character gets a +4 to any active or passive Will saving throws to disbelieve the illusion spell. If it is a spell which has an instantaneous duration, only those who identify it as an illusion get the bonus. If the spell has a duration longer than instantaneous, the fact that is is an illusion may be communicated to everyone near the character who spotted it as an illusion.