The Unknown

As an additional cost to cast The Unknown, sacrifice six lands.
The Unknown is all colors and can't be countered.
Flying, First strike, Vigilance, Indestructible, Shroud
When The Unknown enters the battlefield, each player shuffles the cards they own in exile into their library, then exile all graveyards.
The Unknowns power and toughness are each equal to the number of cards in exile.