Kakuno, the Spellstealer

Flash, haste
Kakuno gets -0/-X, where X is the total
converted mana cost of cards exiled with him.
{3}{u}, {t}: Exile target instant or sorcery spell that
targets you or a permanent you control. You
may cast that spell for as long as it remains
exiled, and you may spend mana as though it
were mana of any type to cast it.