Dreamworld Equilibrium

Whenever a nontoken creature enters the battlefield under an opponent's control, you may search your library for a Memory, Nightmare, Somnid or Thought creature with converted mana cost less than or equal to that creature and reveal it. If you do, put it onto the battlefield, then shuffle your library.
"A dream doesn't become reality through magic? Well, too bad because it frequently happens here on this world." - Dogun