Cthulhu, Endless Dreamer

If tapped for mana Plains produce {u}, Islands
produce {b}, Swamps produce {r}, Mountains
produce {g}, and Forests produce {w} instead
of any other type.
If a player would cast a spell from their hand
they instead exile it and reveal cards from the
top of their library until they reveal a card that
shares a card type with it. That player casts the
card without paying its mana cost, then they
shuffle their library.