Guardian of the Void

Whenever Guardian of the Void attacks, you may
pay {b}{w}. If you do, put a Crumble Counter on
Guardian of the Void and untap it.
When Guardian of the Void dies or is exiled,
remove all Crumble Counters from it. For
each Crumble Counter removed this way, exile
target nonland permanent. If 10 or more
permanents are exiled this way, also exile the
top 20 cards of each opponent's library.