Goleth'Nalok, Entropic Seed

When Goleth'Nalok enters the battlefield, exile
all nonland cards from the top of your library
equal to the amount of mana used to cast it,
shuffle the rest into your library, then put a
number of +1/+1 counters on it equal to the total converted mana cost of cards exiled with it.
Whenever Goleth'Nalok attacks, cast a card exiled
with it at random without paying its mana cost,
then remove a number of +1/+1 counters from it
equal to that card's mana cost.