Hearthstone [Free] expansions are an awesome avenue that Blizzard uses to shake up their online card game and introduce new mechanics and ideas. The next expansion, The Witchwood, has a spooky motif and we just found out a new deck type that is being introduced called Even-and-Odd Cost which has cards themed around the mana cost of cards in the deck. It looks like some classes will have support for primarily odd mana cost decks and others for even mana cost decks. This is speculative at this point but that's the point of the incremental reveals of each of these Hearthstone expansions isn't it" We get to see and evaluate cards based in a vacuum and then once the whole thing comes out we can fill in the blanks and see the full picture that each game update holds for us!
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Some examples we have been shown so far include legendaries Baku the Mooneater and Genn Greymane. Each of these hinge on either building a deck with odd costs or even costs and providing a strong benefit for the effort. Going with Baku and an odd costed deck makes your hero power more powerful and using Genn and and even costed deck reduces your hero power to 1 mana. These cards have a rarely used mechanic called Start of Game so you don't need to play these cards in order to get the benefit they provide.
Some more examples we've gotten for this format are the Black Cat, Gloom Stag, Murkspark Eel, and Glitter Moth. All of these seem very thematic for th...