How do spell effects stack when determining the DC to disbelieve an illusion?

Question

Classic tactic; You use a ghost sound spell to make your Silent Image seem more life-like. It's such a standard tactic for low-level casters, that it's actually pointed out in the spell. That said, exactly how does that work? Does the use of the two spells give a bonus to the DC of one or the other, and if so what's the bonus? Or, would it make it so you have to make both will saves to believe that the illusion is fake?

I ask because I've actually come across such an elaborate version of this tactic that I'm not sure how to handle it. In a 10th level campaign, the group is helping to hold off a siege from a significantly more powerful army. When brainstorming, the rogue pops off "Well, why don't we just summon Cathulu when they show up?" Everyone laughs, then we continue planning. Except the group wizard, who reaches for the core rule book. As an illusion specialist with a back-story focused around making magical defenses for people, he has all the stuff he'd need to set it up. A combination of magical traps rigged to go off in a specific order while the cleric performs a fake summoning ritual. The spells he's using are Mindfog (for mechanical reasons, as well as the aesthetic effect), Major image (to create the greater part of the illusion), Wind wall (to create some special effects), and the big finisher Black tentacles, all set up to make it look like a portal to a dark plane is being opened, and the eldritch abomination itself is about to come through it.

How do I, as the GM, handle that? What kind of check would that involve, to disbelieve? Is there a table for determining situational-modifiers to disbelieve illusions, and what am I looking at in terms of the actual DC?

I applauded his inventiveness, and it's the last-line defence so it's not actually in the group's plan for it to be needed, but for my puposes the big-bad is supposed to come rushing in at the last minute to grab the king and run off so that the adventure can continue to the questing bit. But how is something, even a Dragon (the big-bad is an ancient Black), supposed to respond to having, literally, Cthulu summoned to stop it?

Also, I'd like to not cheap-out and say that the traps and everything just don't work right. As a 10th level wizard with all the feats and skills needed, and adequate time to set it up, it's not only unlikely that something would go wrong, but it would be an obvious GM-intervention if it did. I don't mind extending my authority to progress the plot, but I would hate to downplay with kind of creative thinking by just saying that it didn't work like planned.

Answer 1

First off I want to congratulate the Wizard for coming up with such an innovative plan for stopping a dragon.

All Spell DCs will be based on your Wizard's primary spell DC

So, at base, this is 10 + Spell Level + Primary Casting Attribute Modifier (Int for the wizard). Assuming an Int bonus of +4, the save DC on Mind Fog is going to be 19.

Failing that first save is going to make the following a lot harder:

There is not a modifier for the disbelief save

The DC is going to be 10 + 3 + Casting Attribute.

But your baddie must interact with the Illusion first, before getting a save. So for Cthulhu, it would mean probably coming into contact with the illusion physically. Now a Dragon is pretty smart, and probably going to think that it's an illusion (especially if it doesn't think someone is dumb enough to summon an Outsider to stop it). It could charge Cthulhu head on if it's brash (and actually assumes it's an illusion). Though, safer ways to go about that would be to try Detect Magic and on round 3, you can determine the school of magic. Or even faster is True Seeing which will get past the illusion instantly.

So, your Wizard will not want to have Cthulhu actually interact with the Dragon, otherwise it'll trigger the save.

It should be noted that the Dragon will probably make it's spellcraft roll to identify Black Tentacles, but it may or may not assume that is being caused by the weird outsider and not some illusionist trying to save the King.

To address the Ghost Sound + Silent Image save DC

You'd save against them separately at their different DCs. Though, if you succeed on the save for Ghost Sound, I would count that as "interacting" with the Silent Image, since you will realize that the sound the illusion was making is an illusion.

Edit: As @SimonGill points out, the enemy army would most definitely freak out seeing this, and would probably not get their disbelief save...

Answer 2

The problem with illusions is that it comes down to you determining how they work. You should certainly stay consistent with any rulings you have previously made about interacting with illusions. This may give you some pointers on Illusion magic from a 3.5 perspective.

Frankly though, this plan is awesome enough to just work against the attacking army. You've said that this is a last ditch effort to disrupt the attack, and the goal is to kidnap the king, which will be done by the BBEG flying in at the end. By this point, the army has done its job and sending them packing will be a good reward for the clever planning.

The Dragon is really the only one who matters at that point. What's his point of view? I expect something like this:

They are insane enough to call up a Great Old One? Where the hell did they get that power and why didn't they just fry my army? Crap. Well, I can just slip in while the portal is opening, grab the king and bug out. Then, I can figure out how to deal with a god that is walking the earth.

Answer 3

Basically, the trick doesn’t work well if things are making the saves: two chances to save increase the odds that a given person will make at least one, and once you’ve made one, you’re more likely to be suspicious.

The idea is, instead, to reduce initial suspicions in an attempt to prevent any interaction with the illusion in the first place. No interaction, no save even gets thrown, and your illusion is at no risk of being seen through.

So what you want to do is combine the spells in a situation where interacting with the illusion is either non-trivial to accomplish, puts someone in danger, or is out of the way enough and subtle enough that no one will think to check it. Of course, it’s best to have all three, and as with all things illusionary, the appearance of these things is almost as good as the reality.

Your group’s plan is a pretty ideal situation. They’re doing the “summoning” inside the castle, in the middle of a battle. Interaction is going to involve approaching the apparent Cthulu, and the black tentacles mean that there are going to be plenty of real consequences for doing so. In reality, the majority of the army probably doesn’t deserve a saving throw at all.

But an Ancient Black Dragon is different. He’s nearly a millenium old, and he’s very smart. A 10th-level party is reasonably well-known; an Int 18 spellcaster would likely look into them. He’s probably aware that the wizard is an illusionist, and thus on the look-out for illusions. His caster level isn’t sufficient for true seeing, but even so he may decide to call BS on their summoning.

If he fails his Will save, of course, he’s going to have to behave as if he thinks this thing is real, but that won’t last long since he’ll pretty quickly have the incontrovertible proof that it was fake.

But sending the army running and potentially wasting a round of the dragon’s time? That is a great success for an illusion. Your party can escape or start pounding the dragon with their own (very real) effects during that time.

And you really should give this a very good chance of working. The idea is very clever, and they’ve got a lot of things going for it. Don’t cling too tightly to your idea of how things are supposed to happen. Make sure to prepare at least a little for what happens if the dragon doesn’t get the king.