Barmecide feast

That which pretends or is imagined to be extravagant, plentiful, or opulent, but which in reality consists of little or nothing; that which proves to be illusory or unreal. Taken from the name of a prince in Arabian Nights who offers a feast to a beggar but gives him only empty plates. The money you make on the stock market can end up as a Barmecide feast: you think you're making millions of dollars, and then in the blink of an eye it is all gone.