If it is a simple thing, then the thing becomes real. If that illusion was a person, then you get a Shade.
A Shade is an illusion of a person which believes itself to be real. It is a crafted soul.
It takes time and introspection to create a Shade. Unless you have intimate access to somebody else's soul, the only person who you can make is yourself. Usually, only you are privy to the depths of your personality: your secrets, your desires, your quirks, your thoughts, the important hidden subconscious bits that define personhood. The crafting is a work of inspiration and obsessiveness - everything must be believable, or the spell will fail.
Shades tend to look better than their original counterparts, or slightly worse. Rarely are they exactly the same. Any model can be used to create a Shade, though the most person-like fare the best - it is harder to convince a Shade that it isn't real the more real it looks. The oldest of Shades are completely indistinguishable from their originals - their joints crackle when they bend and every spiral in every fingerprint is just so.
How to tell them apart? Oftentimes you can't. They bleed and eviscerate the same goo as in all of us. There are two ways, however.
First, a Shade cannot survive a Dispel Magic, or enter an Anti-Magic Field. If subject to these spells the Shade will pop out of existence forever. They will simply cease to be. They don't know this.
Second, to question its premise for existing. There is an innate resistance in Shades to never want to think or talk about the subjectivity of reality. If the subject comes up they will avoid it - they'll leave the room, or come up with some excuse to go, or bring up another discussion topic. If someone suggests they might not be real, they will become uncomfortable and defensive, irrationally so.
If they entertain the thought that they may not be real, if they truly consider it even for a moment, they will start to dissolve into nothing. It is very painful to experience and to watch - a positive feedback loop of destruction. The soul may be illusory, but the screams are real. A Shade, though an illusion, is as much a person as you or I, and that is the worst part of all.
Shades in the World
Nobody thinks, from one moment to the next, that they are aging; therefore, Shades do not age. This can be their downfall. Old Shades will rationalize this somehow, even creating self-believing fictions of immortality or vampirism to shield their minds from the potential truth.
People fear it, for it reveals the superficiality of their existence. The gods hate it and will seek to destroy both it and its creator, for soul-making is the domain of gods alone. Beasts ignore it, for they only concern themselves with the divine spark, in which the Shade has none.
Powerful Shades can learn illusion magic, just like their creators. In fact, they often tend to be better at illusions than those who made them. This brings up the strong possibility that a Shade may create a Shade - a simulacrum of a simulacrum - man making machine making... something. What will be the souls that Artificial Intelligences make? The answer to that is what a Shade's Shade is.
Souls born from lesser inspirations are often not long for this world. They often live just long enough to see themselves, then they melt. They are defective in obvious ways: perfect skin, unnatural flexibility, lack of smell, or a scent too consistent. They are popular with recluses, who create bubbles of fiction in their domains to sustain their creations. Lesser Shades rarely survive contact with outsiders, and so they're often kept isolated.