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The PCs this week tried to do something that stumped me this week, though I am sure this question has been asked before. Caught between a rock and a hard place, the PCs decided that they would cast an illusion spell to try to catch the enemy off-guard. Thus far, everything made sense.

However, other questions began to be raised: illusion spells are generally 'will disbelief, if interacted with', so presumably the enemy wouldn't be given a will save until they interacted with it. As well, the PCs started to ask if they could use it to flank, and generally other things that would happen if the enemy believed the illusion (would it provide cover? Threaten squares? What happens if it is attacked?).

So how do illusions work, game-mechanically, when someone believes an illusion? I tagged pathfinder, 3.5, and 3e as I assume the same (or similar) ruling would apply to all three.

EDIT

Considering the illusion school covers a variety of spells, I am primarily concerned with figments; other sub-schools seem better defined. E.g. silent image, minor image, major image and the like.

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Could you specify the spell? It might matter, because illusion spells are further qualified as figments, glamers, patterns, phantasms and shadows. –  cr0m Jun 21 '11 at 16:50

5 Answers 5

up vote 9 down vote accepted

As far as Pathfinder is concerned, illusions do not threaten squares. Pathfinder even has a Metamagic Feat that can be applied to figments in order to make them threaten a square, Threatening Illusion (Metamagic Feat) and the "normal" situation is called out at the end of the text on that page.

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Moderators/editors, I propose that my answer be rolled into Cthos's as an addendum, since his is more comprehensive and addresses multiple editions per the Question. –  cr0m Jun 22 '11 at 16:34

The following are all Pathfinder-related answers and apply to the (Figment) Subtype:

Illusions are real until you make your will save to interact with them. Here are some example of what they don't do, as far as figments go:

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.

Further, here are the rules for what happens if you don't disbelieve:

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.

Based on these things you can extrapolate how an illusory combatant would operate (this is based off of GM Noob's answer with futher clarification and interpretation):

Could it be used to Flank

No, because you must threaten a square to participate in a flank.

Does it Provide Cover

Yes, probably. The idea here is you're trying to aim around something, which is making your target higer to hit (+4 to AC). Note this is a soft cover bonus, so you don't get a bonus to Reflex Saves.

Does it threaten Squares

No, it does not technically threaten squares. Also keep in mind that most illusions require the caster to be concentrating for them to react.

What happens when it's attacked?

For pathfinder anyhow, it's straight out of the illusion section:

A figment's AC is equal to 10 + its size modifier.

And from Major Image:

The image disappears when struck by an opponent unless you cause the illusion to react appropriately.

Edit: Some illusions do indeed have hit points, but I believe those are all of subtype (shadow), which means that they follow different rules.

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Really great answer! –  oldrobotsneverrust Jun 22 '11 at 13:55

We always played that illusions are 100% real until disbelieved. Even for other pcs. So yes all those things apply

1. could it be used to flank?
Depends on the spell school sub type

2. would it provide cover?

Yes

3. Threaten squares?

Depends on the spell school sub type

4. What happens if it is attacked?.

Some spells have HPs, some dissapear as soon as they are attacked, some just allow a save attempt when they are interacted with in anyway.

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We always played it depended.
If they believed they were on fire then they took damage(though it all healed after a standard rest or with any healing spell).
If the illusionist was there an incontrol of his illusionary dragon the player feels the resistance as he tries to stab with his spear. If he believes the green dragon blood spilling on him is poison he feels the effects. If he believes that the stone bridge is real then he will perceive any slight change in angle of the ground as part of the bridge. But once they attepmt to step out into nothing they plunge through. An Illusionary door may be opened but the stone wall that is actually there will not yeild to the players passage. A giant hidden by a camoflage spell will still appear to be a small hill to the player right up until it is dispelled when the club swings at their head.

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I'd play it using the "if you believe it's real, you treat it as real until physics tells you otherwise". Speaking as a DM ruling off the cuff:

Can you use it to flank? Yep - flanking is "your opponent has to divide attention between multiple threats". So long as your opponent thinks that's a swordsman behind him, anyway.

Would it provide cover? I'd say this is a case of physics trumping - an illusion won't stop an arrow. I would have the NPC act as if it did (moving to clear the cover, for instance), but if he shot into cover and he would hit if there wasn't cover, he'd hit and get a save (since "arrow flying through monster" is a good hint of illusion)

Threaten squares? I'd say no, but simply because illusions don't generally have the free will for such things. Again, I'd have the monsters worry about it (5-footing away, for instance).

When it's attacked? No matter how it's attacked, I'd treat it as a touch attack. (If you miss the touch attack, then you didn't actually physically connect with it, and wouldn't give you a reason to doubt). If you hit, then I'd probably dispel it immediately (the sword slash disrupts the magic-y bits), unless the illusion is a monster that you would expect sword slashes to not touch. (Ghosts, say).

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