The Lich King is being very naughty and is being disabled in Battlegrounds.
The issue? If you used his Hero Power to give a minion reborn and then make that minion a triple, the minion will have reborn for the rest of the game. We don't mind sharing this bit of info because if the Lich King is still active in the rotation, 17 hours from the original blue post, others are going to be using this to their advantage and it'll put you on a level playing field.
We’re in the process of temporarily disabling The Lich King in Battlegrounds due to an unforeseen issue. We’re expecting to get the issue fixed and have The Lich King reenabled in our next major patch. Thanks for your understanding.
Update: It’s going to take a little longer than originally anticipated to disable the Lord of the Scourge. We expect The Lich King to no longer appear in Battlegrounds matches within the next ~24 hours.