Illusion

"Belief is the keystone of illusion."- Vhagar

Sorcery medium which is intended to be extremely easy for others to claim, used to 'infect' other souls in order to make them perceive various effects. Illusion is one of the 'softest' applications of arcanism-like practices, its projection being primarily a barely-coherent anchor for Celestial quanta in order to primarily affect souls. The medium itself is sometimes referred to as the "illusionist's brush".

Functionality

Once an illusion has been absorbed by a claim, whatever effects it applies can work without the restriction of having to overpower a claim. Illusion can more efficiently be generated by drawing from the Plane of Darkness, around 2-3x more output- quite a bit more technically challenging though. This isn't very helpful, seeing as technical complexity is often overwhelmingly the bottleneck in cast illusions. It's incredibly easy to break an illusion once truly recognized, and reinforcing it against such would take orders of magnitude more energy, not just a 300% increase.
Visibility, projected illusion takes the form of a transparent, fuzzy-looking projection, grayish by default. Color can be modified by producing ordinary light (which by default is the color of one's natural light color) and allowing it to diffuse through the medium. The medium is extremely weak and practically intangible- in no way can it actually be used to create a physical obstruction or weapon. It doesn't even strictly exist on the physical plane, being more shaped by the Significance infused into actions through the soul-force effect than physical force, floating in loose superimposition. As a result, even though it can easily enter an opponent's claim, one cannot simply use this to inflict physical damage. Instead, it is used to manipulate or deny senses in ways only the target perceives- illusions. This intangibility is also a key factor in why sorcerers can produce it with so little energy. However, it requires very high levels of precision in order to create usable effect as a result.
It is important to remember that whatever effect is created with the illusion will also affect the user until it has been claimed by another. Adding a time delay to effects is a common workaround to this, though it makes sustaining and creating the spell far more stressful- more pronounced effects are generally more volatile, and can be delayed for less time.
The illusion creates its functions by forming into fragments of memories (remember that memories in this world are coherent magical objects utilized by the soul) affecting certain senses, and being subsumed by a new claim integrates those memories, creating the experience. Shaping a memory requires much practice and study- creating a 'garbage' memory and attempting to implant it will not cause a seizure as one might hope. Instead, the target's soul will simply reject it.
Large projections can also be created with illusion, although they are rarely capable of fooling anyone due to washed out color and intangibility unless provided with significant amounts of power. In a low-visibility situation (ie, not with Cotd opponents), there is some potential for use there. A certain Shrouded utilizes large projections made up of many smaller pieces, applying effects to anyone passing through the area. This has an impractical energy cost to ordinary Cotd.

Learning

Strictly speaking, illusion can be learned by non-Cotd. There are two main stumbling blocks to this: first of all, it is only possible to clearly see the illusion's internal structure with darklight vision- it simply appears fuzzy grey in normal light. While theoretically possible, it'd be very hard to actually do anything without a Cotd helping, and a lot of time would be wasted with attempting things that simply wouldn't make sense if one could see what was actually going on. Second, making a weak claim is much easier when Empty, and increasing levels of soul flow make it progressively more challenging. At the same time, it requires a lot of the same skills as any other high-level arcanism, being a step up from solid shadow. Therefore, the only ones with the skillset to learn it are the ones who are the least able to make it work in an effective setting due to having performed flow-enhancing rituals. As a result, 'illusion' outside of the Cotd may make use of the medium, but purely to create large, intangible projections without any special effects.

Targeting

Most illusions target the eyes, even if they actually affect other senses. Eyes being the window to the soul, they are the most receptive to soul-binding effects.
Illusions share the same flight speed limitations and energy costs as other projections.