Hearthpwn

Shadowform? Check.

Velen? Check.

My TGT version of the deck I used to play  some time ago, adapted to the current meta.

Against aggro(cancer pallys, hunters, casino mage, mech mage) mulligan for Zombie Chow, Wild Pyromancer and Shadow Word: Pain. If you alreday have zombie chow or pyromancer in starting hand it is ok to keep Power Word: Shield. You won't be playing any cards other than these, in the early game. Drop a tempo Shrinkmeister without thought if you don't anything else. You should even consider keeping it on starting hand(I would keep against hunters for sure). Against hunter and cancer paladin the strategy is about the same; you want to drop something early and use pyromancer combos to clear the board until you estabilize with Cabal Shadow Priest or Holy Nova. You'll use Tournament Medic to get back your  hp if you're too low. Against tempo mage you are going to do the same, but usually in other priest decks you'll need some good aoe, which you don't have here. You'll go for board presence until they run out of gas and you clear with holy nova or Lightbomb.

Against druids you should mulligan for Shrinkmeister and Shadow Word: Pain. Here all you want to do is stall. Lay a big Lightbomb is the way. Shrinkmeister is  a mvp in the druid matchup geting a REALLY strong tempo play with Shadow Word: Pain or Cabal Shadow Priest. Save them for a big play, refrain from droping them without a combo.

Against patron warrior you mulligan for Shadow Word: Pain to kill their Acolyte of Pain on turn 3(I would even go as far as keeping Lightbomb). Here you'll drop everything for tempo as early as possible, so they can't get confortable enough to draw their entire deck quickly. Lightbomb is the card need to win the game in a board full of patrons. The lose condition, here, is a charging frothing to your face(basicaly the same losing condition that any other priest deck has). First Warsong Commander will eat a Lightbomb, the second will kill you. If neither of these kill you, you win.

Against demon/handlock you'll want Shadow Word: Death(for Mountain Giant) and Shrinkmeister + Shadow Word: Pain combo(for Twilight Drake). Lightbomb is a good card too, but you shouldn't keep it in starting hand. Here you lose if you can't answer their first treats. If you do, you win by removing and answering their plays and using Mind Blast to finish them with Shadowform damage.

I don't see any other decks in ladder so I wrote about the ones I face often.