Oath of Karn

When Oath of Karn enters the battlefield, exile
the top card of your library face down. For as
long as a card remains exiled this way, you may
look at it, you may cast it, and you may spend
mana as though it were mana of any type to cast
that spell.
At the beginning of each end step, if a
planeswalker entered the battlefield under your control this turn, exile the top card of your
library face down.