Nightmare of Insanity

When this card enters the battlefield, exile the
top card of target player's library face down. At
the beginning of target player's draw phase, look
at the top two cards of that player's library, exile
one of them face down, and put the other back
at the top of the library. You may cast any card
exiled by Nightmare of Insanity for as long as
they remain exiled, and you may spend mana
as though it were mana of any type to cast
those spells.