This is the Neviilz Prophet Velen OTK Priest Deck he used on stream to 
climb the ladder. I’ve played around with it myself now and it’s incredibly fun.
It can take a while to get all the combo pieces in-line, but once you do you just blast your opponent for load of burst damage.
Prophet Velen doubles the damage of your healing spells. So when you use Embrace the Shadow or Auchenai Soulpriest, you can start using your healing spells to do double damage.