Since the whole Mythic RtRtR Edition from Hasbro's Toy Shop was a clusterfuck, WotC apparently send out this apology letter and pack that has one of 40 foil promos with extended art:

The letter

Current list of known cards with pictures

There's speculation that this "preview" refers to a Masters set for December that has been rumored for a while now, but that part is pretty vague.

Edit: Summary on MTGGoldfish with a spoiler gallery