Niv-Mizzet's Diary

Imprint - Whenever a player cards an instant or sorcery spell, exile it as it resolves. At the beginning of your upkeep, you may copy an imprinted card and cast the copy without paying its mana cost. It can't cause you to take extra turns. Then, if there are three or more cards imprinted on this artifact, sacrifice it and create a token that's a copy of Niv-Mizzet, Firemind.