Nemza, Lord of the Necrosages

Flying.
Whenever you cast a spell you don't own, you get
an experience counter.
Whenever Nemza, Lord of the Necrosages deals
combat damage to a player, look at the top X
cards of that player library, where X is one plus
the number of experience counters you have,
exile one of them, then put the rest into their graveyard. You may cast that card for as long as it remains exiled, and you may spend mana as though it were mana of any type to cast that spell.