The latest Hearthstone expansion, Knights of the Frozen Throne, introduces the Death Knight Hero cards, which are currently the staple in the meta. They seem so powerful that Blizzard decides that they will not be available in the game’s Arena mode from November 14 onwards.
According to community manager Zeriyah, the cards are exciting and powerful to play, but their permanent Hero Power upgrades are hard to combat in a format where answers are limited. These cards will not be removed from existing drafted Arena decks however.
The company will still monitor Arena balance and “make changes on an as-needed basis.” Meanwhile, you can try drafting them in Arena from now until tomorrow-ish.