This is the Mana Bind HS Mage Card from the Journey to Un’Goro expansion due for release in April.
It’s a Mage class spell and the card reads: Secret: When your opponent casts a spell, add a copy to your hand that costs (0).
You can get Mana Bind by opening packs from the Journey to Un’Goro set.
*Good to see a new secret that might see some play.