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They've recently announced that Ysera the Dreamer will leave Core later this month, and once she does, it’s going to become way more expensive to solve the mystery of Castle Nathria and earn the Deviate Delight cardback. Here's a great guide to how it's done:
https://www.hearthstonetopdecks.com/hearthstone-castle-nathria-secret-achievement-guide/
You do have to be careful to use the correct version of cards that exist in both an old expansion and the current Core set. CMac on the official forum helpfully notes:
Make sure each card is the version FROM ITS SET, not the core version! - The only two core cards are Ysera and Noz. - Every other card should come from its own set. - For example: for Tar Creeper tab to the Un’Goro button on the side to select it, don’t search “Tar Creeper”, or it will default to the “Core” version. - Make sure to click each set name to pick the cards, don’t use search. - If you’re still confused, look for the icon on the back of the card, underneath the game text. “Core” = a wolf icon. Make sure it’s not the wolf icon, and is the icon for the HS set it came from (Except for Ysera and Noz, which have the wolf/core icon).
I was lucky to finish this one prior to last year's Core rotation when it was more F2P-friendly but I wanted to help spread the word. Let me know if you go for it!
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