Void by Skulkor
What if you could plant an idea in someone's head, giving them memories and create reality?
Have the spectator choose a card, and holding on to it so no one can tamper with it. The moment you pull the information out of their mind, the card turns into a blank card...and so does the rest of the deck!
The human mind is fragile, there's a fine line between imagination and reality.
This is exactly what Void does.
It alters the very fabric of reality that your spectator believes.