Quote from Zeriyah

Death Knight Hero cards can no longer be drafted in Arena.

Starting on the morning of Tuesday, November 14 PST, we will remove the nine Knights of the Frozen Throne Death Knight Hero cards from the Arena draft pool. While the Death Knight Hero Cards are exciting and powerful to play, their permanent Hero Power upgrades are hard to combat in a format where answers are limited.

We will continue to monitor Arena balance and make changes on an as-needed basis. Please note this change does not remove Death Knight Hero cards from existing drafted decks. For a complete list of cards that are not available in the Arena draft pool, please refer to our Arena Specifics forum post on the official Hearthstone forums.