The Multiverse

Whenever a card is exiled, it gains Reemergence X,
where X is equal to its mana value. (This card is
exiled with X time counters on it. At the
beginning of your upkeep, remove a time counter from
it. When the last is removed, shuffle it into its
owner's library.)

Whenever a player casts a spell from hand, that player creates a copy and exiles the original spell.