Illusion appears in the manga and anime.
Illusion is the last card captured in Volume 1 of the manga. It can appear to be different things to different people, but to Sakura, it appeared as her mother. This was very upsetting for Sakura, and she didn't understand at first that it was really a Clow Card, and was almost seriously hurt twice because of Illusion. Luckily, the second time she faced the card, she was able to see the Illusion pattern under the apparition. She fought the card with Watery, and changed it back into a card.
In the anime, the story was basically the same as the manga.
Illusion is a powerful card that shows the person that looks at it what they most want to see. Sakura uses this card to catch the Sword card later on.