Redesigning Illusions: Or Am I?

[virgil]

Free-form magic is a recipe for disaster, as it requires a level of MTP that guarantees divergent expectations of what you can do. Ars Magica suffers from this, and Mage is non-functional for the same reason. D&D avoids much of this by using explicit functions for the majority of its magic, and the open-ended exceptions are by-and-large some of the most powerful spells in the game. Illusion is particularly bad if the implications are taken to their logical extreme. So much so, that most DMs will stealth-nerf them all over the place.

The rules for interaction and disbelief are another persnickety element, and need to be more explicit.

As mirror image and invisibility both work as illusion spells, it sounds like the answer is to figure out the specific tricks of the illusionist and then build either a system or a set of spells that fill those effects. These are the explicit goals I can think of for using illusions...

• Terrain Addition - Fake walls, pits, rocks, etc. Most are for hiding inside, but the false bridge is a classic.
• Disarm - Turning swords into snakes. Distracting them to release their grip, which works even better while climbing.
• Threat Enhance - Make a man into an ogre, or an attack dog into Cerberus. The more advanced ones will make friends look like foes.
• False Threat - Illusory monsters to make people act defensively at thin air.
• False Friend - Semi-sentient illusion that will hold a conversation, ranges from a projected image to semi-independent herald to full-on simulacrum, with a semi-independent quality of how 'real' they are; easily rolled up into the False Threat option
• Disguise - Light one to make guards not question you and a more advanced one where you can sleep with a person without them noticing; an offensive one used on others can strip them of their social status
• Glamour - Similar to disguise, but makes you do better at the social mini-game. Easily expands onto the inanimate to make leaky sheds seem as gilded mansions
• Mindscape - A more elaborate version of sleep, but for some reason you can't just CdG them; a lesser version essentially blinds them
• Scarecrow - Longer duration, minimally active such as a sleeping double or even an endlessly repeating guard on patrol
• Sensory Denial - Remove a sensory quality to make the target undetectable to that sense, the more advanced the magic becomes the more difficult it is to find them
• Messages - From holographic plans of the Death Star, to Chocolate Frog cards, and all the way up to magic mouth
• Unnerving Gaze - Kytons use it, the trick of showing someone a loved one (especially a dead one) to make them freeze in place, open themselves up for an attack, or just plain not attack the figure

What other 'tricks' can you think of that should be on this list?

[Previn]

Does your definition of illusion cover phantasms and shadow-stuff?

[Laertes]

For something as broad as illusionism, trying to come up with an exhaustive list seems like a fool's errand. This is especially the case if your game is larger than merely combat. However, a few others for your list:

Mass entertainment
Mass education
Sensory denial (create an illusion of a wooden box around the person, moving as they do. Hey presto they're blind!)
Comfortable quarters (it might be a stable but it feels like a palace)
Sordid sexual fantasies of various types

[virgil]

Phantasms, certainly, since that's only a matter of whether one person perceives it or everyone does. Mindscapes are inherently this category, for example.

Shadow spells are an interesting quandary. In a lot of ways, they're just conjurations and evocations (or even transmutations with the threat enhance or false threat options) that can get disbelieved.

[Laertes]

Does remote viewing count as illusionism? Do non-visual stimuli count as illusionism? Can you, for example, flood a person's brain with happiness or with pain?

[virgil]

For your first post Laertes, I don't see the need to do the moving box thing, as I've only really seen that interpretation to take advantage of the open-ended nature of silent image. Just call it Sensory Subtraction; either removing the trait from someone (invisible or silent), or removing the sense from the target (blind them or deny them the smell of freshly bathed baby).

In the case of making the outhouse a pleasure palace, I should treat that as an expansion of the terrain addition trick, possibly even glamour on the environment. The difference between messages and mass entertainment/education is a matter of scale.

Does remote viewing count as illusionism? Do non-visual stimuli count as illusionism? Can you, for example, flood a person's brain with happiness or with pain?

I'm sticking to the stage magician style of illusions, only manipulating the senses insofar as your environment is concerned. Once you go down the rabbit hole of the illusion of love, you're covering effects more associated with mind control; and remote viewing is associated with divination, not illusionists. I'm already pushing the envelope by tying Unnerving Gaze to illusions, but it's a trope even shapeshifters use if they know their victim's backstory enough to take the form of their lost sister.

[tussock]

Your illusion spells just need to provide a set of conditions and damage functions the same way as any other spell. We don't actually get fussy about how fire works, it's d6/level or whatever, you don't need to get fussy about illusions either.

So you can illusion up a summoning spell, only the monsters have AC 10 and 1 hp doing subdual damage with a save to dispel. Or illusion up a damage spell for d6/level subdual, save for none. Or illusion up objects with 0 hardness and 1 hp. Fear effects, slow, stun, distraction, sickening, attraction, repulsion, charm, whatever.

Infact, just let illusions recreate arbitrary spell effects with extra saves and subdual damage and short durations or whatever. So you can cast charm person or charm a person with an appropriate illusion of something they're already loyal to.

[Laertes]

It depends on how murderhobo your game is. Once you start looking into things like leadership, social interaction, stealth and even ruling a kingdom, illusions become vastly larger in scope and you'll need a system for determining it.

For example, one of the most important trade goods in real history was spices. Spices aren't vital for survival: their function is merely to make things taste nice and disguise the taste of off food. This can be done with illusions. Therefore, the illusion industry could be at least as profitable as the spice industry was.

[angelfromanotherpin]

I think that the illusion system would work best as an extension of a workable Bluff system. That is, once you have a Bluff system you're happy with, illusions allow Bluff checks that would otherwise be impossible.

[fectin]

Disguising passageways.
HUD-like info overlays, either for one person or actually overlaid.
Designation.
Illumination.
Invisibility.
Stagecraft.