Vither, Forge of Universes

At the beginning of your upkeep, scry 1, then draw a card.
Supernova {u}{u}{r}{r} ({u}{u}{r}{r}: This creature becomes an enchantment and loses all other types. It gains the subtype Star.)
If you would draw a card while Vither, Forge of Universes is a star, instead exile the top three cards of your library. You may cast those cards this turn.