When this is the case in any Trading Card Game, the cards are known as Secret Rares
You can watch videos of people opening sealed booster cases and boxes to verify this for yourself
The selection almost always includes a mixture of brand new cards (where the card name, in-game rules, and art are all new) and 'reprints' (new versions of existing popular cards, which keep the same name and rules, but may feature new artwork and sometimes updated wording)
Epic cards will be alternate versions of existing cards, with numbering that goes above the base card numbering, similar to Enchanted
It will contain 204 cards in the main set and every single one will have a foil and non-foil version meaning 408 for every foil and non-foil in the main set
The Enchanted variant is looking fairly valuable as well, currently sitting at around $190 market price, while its Legendary is fetching around $6-$8 right now as well