Peaceful Plateau

Peaceful Plateau enters the battlefield tapped.
{t}: Add {w} or {u}.
{x}, {t}, Sacrifice Peaceful Plateau: Add X
mana in any combination of colors to your mana
pool. Create a 0/1 white and blue Human
creature token with "At the beginning of your
end step, if no creatures attacked this turn,
you may return Peaceful Plateau from your
graveyard to your hand."