Whenever you cast a Demon, Devil, Wizard, Druid or a card with an occult symbol in the artwork, put a quest counter on A Sin Highway.
As long as A Sin Highway has five or more quest counters on it, it becomes a 12/12 Demon with "At the beginning of your upkeep, make a sacrifice, or you die. Your library is black."
For Challenge # 025, spinning off of Aysen Highway, because I really couldn't resist.