This is The Caverns Below HS Rogue Quest Card from the Journey to Un’Goro expansion due for release in April.
The card reads: Quest: Play four minions with the same name. Reward: Crystal Core.
Crystal Core
This is the Crystal Core HS Rogue Card from the Journey to Un’Goro expansion due for release in April.
It’s a Rogue class spell and the card reads: For the rest of the game, your minions are 5/5.
You can get The Caverns Below by opening packs from the Journey to Un’Goro set.
If you want to know more about how the Quest keyword works check out this post.