Aphir, Runic Archmage

Whenever a Rune enters the battlefield under your control, exile the top card of your library. Then put a casting counter on Aphir, Runic Archmage.
Whenever you sacrifice a Rune, you may cast a card exiled with Aphir, Runic Archmage with converted mana cost less than or equal to the number of casting counters on Aphir.