Mind Barrier

Whenever one or more creatures attack you or a
planeswalker you control, scry X, where X is the
number of attacking creatures. Then, reveal the
top card of your library. If it's a nonland card
with mana value equal or less to the number of
attacking creatures, you may cast it without
paying its mana cost. Otherwise, draw a card.
{1}{w}, Exile Mind Barrier: Creatures can't attack
until your next turn. Activate only as a sorcery.