maybe im late but i noticed that some of the cards when shown in the deck have a different mana cost than in game. then i switched my herald to a different faction and i noticed that those not in the same faction as the herald had higher mana cost while those that were in the same faction had lower costs. if theres a thread with all of this kind of stuff could someone link me?
Your herald determines the main faction you're using. All cards have their respective faction alliance under the level rank. If that's not enough, there's a background symbol behind every card, also showing their faction (purple = demon, blue = kingdom, orange = dragon).