Illusion is the school of magic that involves messing with perception - creating phantom sights and sounds and similar things, hiding things and changing their appearance. Some of the simpler applications may even manipulate light and shadow, aiding the user in basic camouflage and invisibility is a common application of this school as well.
The simplest interpretation of illusion magic involves what are basically magic holograms - projected images lacking in any real substance that either move according to pre-programmed instructions or in accordance with the will of their controller. Anyone can see them, they look the same to everyone and they look as real as the creator can make them. Phantom smells, tastes, sounds and sensations are more complicated … some of these are difficult to express without actually creating something (an image of a dragon, after all, is nothing but an image, but an image of a dragon's roar doesn't differ much from the original roar or from a recording thereof). Resisting this sort of illusion is mainly a result of noticing discrepancies in the image (the dragon doesn't have it's characteristic smell of sulphur, it's flapping wings don't generate a breeze … it doesn't leave footprints). In some cases, especially illusory walls, these images may only be visible from one direction.
More complicated still are those illusions that exist mainly in the mind of the viewer (and potentially overlap into charm magic) - those that effectively tell the viewer's senses "there's a dragon here" and let them provide the balance of the information themselves. These would tend to be all or nothing attacks - either the target's mind plays along or it plays up (either because it replies "no there isn't" or because it 415's the request because it doesn't know enough about a dragon to create a credible image … although the attempt could still cause some sensory disruption whilst reality is restored). Also, different viewers may see different things. Resistance to these is likely to run the gamut of available methods - although the viewer's mind may spend some time patching discrepancies. In some settings these illusions can make their victims pass out or suffer psychosomatic harm (up to or including death) due to their perceived reality1.
Some systems even allow illusion magic to include solid or semi-solid constructs made of "hard light", "congealed shadow" or even ectoplasm (although this may clash with conjuration and/or evocation), which are at least quasi-real and resist the "poke it with a stick" test … sometimes violently. Their ability to interact with the physical world is likely to vary greatly, with only the most advanced being able to handle complicated tasks like combat without direct operator control. The creation of fetches may fall within scope of illusion magic.
And then we have invisibility. Quite a popular goal for humanity, but not easy to achieve even with magic. The "type 1" version should probably be expected to hide the user by projecting around them the image of the world without them in the way. This is likely to be the easiest technique, and under some circumstances may even work. Likely the better the worker, the tighter the fit, but there still may be problems along the "seams" of the image. This variety is likely to be effective against electronic surveillance as well (but perhaps not across the entire spectrum) but some versions may look odd when viewed indirectly. Discrepancies in the image will likely be dependent on the skill of the worker. The "ignore me, I'm not here" type would seem robust, but is prone to fail abruptly if anyone fails to be effected and likely shows up clearly on recordings. Depending on the magic involved, this may not work at all on animals. Animals in particular may also react to a person they can smell and hear but not see. The best sort of invisibility probably involves bending light around the user - but this may make it difficult for them to see in turn.
The fae are famously fond of this sort of magic (indeed it may be their main or only skill in the art) and are well known both for wrapping their true selves in it (commonly called "glammers") and for creating whole illusory environments (up to and including virtual food and drink).
Game and Story Use
- This is a school with a lot of depth to it, from entry level workings that are little more than folk magic - for example a thieves' charm to wrap the user in shadows - to multisensory masterpieces that can kill you before you realise they aren't real.
- Kill someone with an illusion? How about creating an illusory fire in the room to drive them out onto the fire escape … which is also an illusion? A false floor over a big hole? Neither of those need them to play along … but having them die of fear when an illusory dragon breathes on them, or binge on illusory food and drink until they die of thirst or hunger, or eat broken glass thinking that it's sugar-candy … that's fae level work.
- Actually, into the modern era, illusory food might end up being a diet fad with those rich enough to hire a skilled illusionist able to eat to their hearts content without actually taking on any calories. Medical assistance might be advisable.
- In a wainscot setting, the robbers that are recorded on CCTV helping themselves in front of oblivious staff are probably using type 2 invisibility. The ones that are a murky blur or don't appear at all are using 1 or 3.
- When a PC resists an illusion, don't just say … "meh, it's an illusion alright", try some flavour - perhaps they notice a moth fly through the illusory ogre's chest, or that the apparent fire isn't giving off any heat…
- Other examples of failure may include a projected image not reflecting properly in a mirror - or the target's brain not allowing for the illusion in a reflection, revealing whatever is actually behind the illusion - which may be nothing, or may be whatever it really looks like beneath the glammer.
- A badly botched illusion may even end up projecting an image of the creator or something based on an intrusive thought that disrupted the casting (a gas cooker left on, a pet waiting by its food bowl, Chloe Vevrier…). Which might well be played for laughs…