This is the Swamp King Dred HS Hunter Card from the Journey to Un’Goro expansion due for release in April.
It’s a Hunter class Legendary Minion and the card reads: After your opponent plays a minion, attack it.
You can get Swamp King Dred by opening packs from the Journey to Un’Goro set.
* An interesting mechanic we haven’t seen before. One of the better Hunter legendary in recent expansions.