Marionette Design is an ability the possessor of the Millennium Key can use to redecorate a person's soul room in order to change their personality. Their personality can be restored to normal afterwards by having them touch the Millennium Key.
Shadi entered Yugi Mutou's soul room after discovering that Yugi had solved the Millennium Puzzle. He wound up playing a Shadow Game with Dark Yugi, as he searched for Dark Yugi's true soul room. He warned Dark Yugi that if he did find his true soul room, he would be able to redecorate it. However it did not come to that.
Shadi planned to put Professor Yoshimori on trial for excavating a tomb in the Valley of the Kings, which he regarded as defiling the territory of the Gods. Before putting him on trial, Shadi entered his soul room and discovered that Yoshimori was expecting a visit from Yugi and his friends. Instead of putting Yoshimori on trial, Shadi used Marionette Design to turn Yoshimori into a zombie puppet whose only intention was to make Yugi's friends suffer. He hoped this would cause Dark Yugi to appear, allowing Shadi to test him.
Possessing Yoshimori alone was not enough to bring out Dark Yugi, so Shadi entered Anzu's soul room. Seeing that Anzu was an innocent person, Shadi felt some remorse in possessing her, but regardless thought it was necessary to draw out Dark Yugi. He at least decided to spare her the dignity of being a zombie and turned her into a doll who would stand motionless unless he commanded her to do otherwise.
Dark Yugi returned Anzu to normal, by defeating Shadi in a Shadow Game, the Trial of the Mind, causing the Millennium Key to slide down to Anzu's hand. He instructed Jonouchi to touch the key off Yoshimori to return him to normal too.
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