Renia Adler, Rogue Agent

Whenever one or more creatures you control deal combat damage to a player, exile the top card of
that player's library. For as long as that card
remains exiled, you may cast it and spend mana as though it were mana of any type to cast it.
Whenever you cast a spell you don't own, put a
+1/+1 counter on Renia Adler, Rogue Agent.
Discard a card: Renia Adler gains double strike
until end of turn.