Niv-Mizzet's Diary

Imprint — Whenever a player casts an instant or sorcery spell, exile it as it resolves. At the beginning of your upkeep, copy a card exiled with this artifact. You may 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 exiled with this artifact, sacrifice it and create a Niv-Mizzet, the Firemind token.