https://www.pcgamer.com/hearthstone-designer-reveals-new-the-witchwood-cards-for-even-and-odd-decks/

Hearthstone players will soon be headed off to The Witchwood, wherein they will discover 135 new cards, a new Dungeon Run-styled mode called Monster Hunt, and as lead designer Dean "Iksar" Ayala talks about in this new video, an interesting twist on deck building. 

Even-and-Odd Decks are enabled by two new legendary cards that were revealed as part of The Witchwood expansion announcement. Baku the Mooneater upgrades the starting hero power for players who have a deck comprised exclusively of cards with an odd cost, (which means you'll get same hero power as Justicar Trueheart enables), while Genn Greymane reduces the cost of starting hero power to just 1 Mana for players who have a deck built of only even-cost cards.