This is the Corrupting Mist HS Warlock Card from the Journey to Un’Goro expansion due for release in April.
It’s a Warlock class spell and the card reads: Corrupt every minion. Destroy them at the start of your next turn.
You can get Corrupting Mist by opening packs from the Journey to Un’Goro set.
*Interesting cheap removal, it will corrupt your own minions too though.