There is only one reward card (out of 9 chests) for the current season end rewards, but it is huge! (2021.10.16)

What could it be?

Yes. A Rare Dragon Monster card. And not a simple Rare Dragon Monster card, but a Gold Foil card. A Level 2 Gold Foil Twilight Basilisk.

This card currently has only 2404 cards in circulation, and this card currently worth $20 USD.

But the card itself does not really look that useful in the game because of its low (3) Health and low (1) Armor.

And my max tier is still Bronze I.