The Illusion has the ability to make a person see what they expect to see, what they want to see, or what the card's master wants a person to see. The Illusion is suprisingly malicious and agressive, though the reason why is unkown. It has no main body and was seen captured while inbetween transitioning between two forms, which looked like a kaliedoscope-patterened sillouhette of a person. The Illusion is under The Dark.
The Illusion first appears in Episode 6. Sakura and her friends had gone into a forest after Naoko Yanagisawa tells them about a bright light she had followed into the forest. As (almost) all of them expected to see a ghost, when they reached the lake, they soon ran from the Clow Card. When they were talking about what they saw, each of the girls described monsters that looked wildly different from one another. Though, despite them all seeing 'ghosts', Tomoyo said she saw a nikuman, a type of meat bun, since she was hungry at the moment. In the manga, Tomoyo saw King Penguin from King Penguin Park as she was thinking about the aforementioned penguin at that moment. Sakura then suspected it was card and returned to the lake that night, despite her fear of ghosts. The Illusion took the form of Sakura's deceased mother, Nadeshiko, to lure her into walking of a ledge. In the manga, The Illusion tried to make her walk into the lake so she would drown. Cerberus attempted to save Sakura, but failed as she had subconciously rejected him from entering the lake because of the happiness she felt being reunited with her mother. Yukito Tsukishiro, though, was able to save her in both the manga and the anime, most likely due to Yue's power. The second time Sakura attempted to capture Illusion, it took the form of Nadeshiko a second time. Sakura was fooled for a while, but when the card attempted to harm her she was disillusioned from a conversation she had with Yukito in that her mother would never want to harm her so the thing in front of her was not her mother. When she realizes this, The Illusion forgoes it's charade and Sakura captures it.
The Illusion appears again in Episode 9. Sakura was chasing after Rika Sasaki after she was possessed by The Sword through a brooch she bought. Sakura uses The Illusion to distract Rika so she would have an opening to capture Sword. In the original japanese, she made Rika see "the person she had the most feelings for" while in the english dub it was "the person she fears the most". In both instances it is their teacher, Yoshiyuki Terada.
The Illusion is transformed into a Sakura Card during Episode 66. It along with The Maze were used by Sakura to make a walkthrough planitarium during the high school festival to cover up her use of magic while trying to figure out where Clow Reed's presence was coming from.