Kassadin, The Void Walker

Kassadin, The Void Walker can't be blocked.
Your opponents can't play colorless spell with converted mana cost 3 or greater. Spells and abilities your opponents control can't
cause you to sacrifice permanents.
{u}{u}{x}{t}: counter target spell with converted
mana cost x or less. if that spell would be put in
the graveyard, exile it instead.
You are null and void.