Severus Snape, Potions Master

Cascade (When you cast this spell, exile cards from
the top of your library until you exile a nonland card
whose mana value is less than this spell's mana value.
You may cast that card without paying its mana cost
if the resulting spell's mana value is less than this
spell's mana value.)
Spells you cast with converted mana cost 6 or
greater have cascade.