Oath's Intervener

Whenever a a creature enters the battlefield under an opponent's control,
Oath Intervener gains double strike until
end of turn.
Storyline {g/w}:(When you cast this spell you may
pay it's Storyline cost, when you do, Search your library for a card with a Storyline cost greater
than this card's, reveal it, shuffle then put on the
top of your library.)