Regent of Empty Light

When Regent of Empty light enters the battlefield, choose a prime number. (The first six
prime numbers are 2, 3, 5, 7, 11, 13 and 17)
Whenever a spell with the converted mana cost
of the chosen number is cast, its owner exiles
that many cards from the top of their library.
Eldrazi spells you cast cost {1} less to cast for each Eldrazi card you own in exile.