Taen, The Omniscient

Retain (When this card enters the battlefield and
was cast from your hand, you may exile it. If you do,
you may cast it from exile for {w}{u} at any time you
could cast an instant.)
When Taen, The Omniscient enters the
battlefield, until end of turn you may cast your
next instant or sorcery spell without paying its
mana cost.