The aura falls off and goes to the graveyard as a state-based action, and your opponent regains control of Purphoros
A strong SST version should answer the buyer question immediately, show a route table, separate official facts from market signals, place product/category/wholesale routes early, and give a risk checklist that protects the buyer from bad purchases
As for story a 6, it actually had a story granted not much of one like it's sequel but it still had a decent story and that was beating your rival to the legendary Pokmon cards and defeating all those who stand in your way too
While the contract is very much not a demon in and of itself, its a five mana Black Enchantment
For example, a card might say, When you have no cards in hand, sacrifice this permanent
The recorded sales of cards for record-breaking amounts and the media's coverage have created a significant amount of public awareness regarding the investment value of trading cards