Witherbloom, the Balancer Mythic, 245 Its unusual to see a standard variant of a card on the list of the most valuable, even if they are at the Mythic rarity level
According to the art, you're magically reaching into a game store and grabbing a booster
Additionally, while this card may demand a rather steep mana cost, as Demons are rather costly, this spell can be strategically cast once you've built up your board with a few Demons, ideally catching an opponent or two who'd over-committed to the board off guard
It's legendary so you can build around it, and its White's answer to Goblin Welder effects which tap to sacrifice an artifact to recur one
There was a lot of hype around this legendary vampire demon noble when it was spoiled, mostly because its one of the meanest cards Ive seen WotC print
Theyre also a great place to get started with identifying strategies, evaluating cards, and modifying decks