One Night in Karazhan has not been released yet, but Blizzard has already banned one of the new cards from appearing in Arena. Apparently, the card is too bad to be used in competitive play.
One of the bigger problems that Hearthstone has right now is that the Priest class is one of the worst classes in the game. Fans had hoped that One Night in Karazhan would bring a few good cards for the Priest class. Unfortunately, they got probably one of the worst cards to date.
Blizzard heard their fans and Hearthstone lead designer Ben Brode answered them. Brode explained why they decided to add the controversial card called "Purify" and why they decided to ban it from Arena.
*The reason Purify is considered a bad card is that it's a two mana cost card, which silences a friendly minion and lets the user draw a card, not the most useful of cards.
Brode admits that they read the community wrong when they added Purify and said that they probably should have added it alongside some fancy Priest power cards. Brode went on to defend Blizzard's decision, saying that some players like to use fun or bad cards and win with them. According to Brode, Purify would cater to those players and could be used quite effectively — though he agrees on that Purify is not a good card, especially not for Arena.
Since Priest is already is a bad class and since this card isn't doing Priest any favors the Hearthstone team decided to remove Purify from the Arena pool altogether, and that's even before One Night in Karazhan has been released.
As per usual One Night In Karazhan will be released episodically with one raid wing coming out per week and it brings 45 new cards to Hearthstone for players to play around with. The first part of this epic adventure will be released later this week and will be free for all players!