This deck is really fun to play, and as the name suggests it relies on completing the quest to then damage enemy with either fatigue damage or things like health cost cards.

Your main goal is to self-damage on your turn either using hero power,  or cards like Flame Imp, Backfire,  Stealer of Souls can also help here. After completing the quest, play Blightborn Tamsin (quest reward) ASAP , next, it's just a matter of survival and board control as every damage you take on your turn will damage your enemy instead.

For the early game you should focus on competing the quest steps, while maintaining at least some form of board control. Mid game aim to finalize the quest, and late game is all about the final push. Try to leave combos like Tamsin Roame + Backfire for the later part of the game obviously. Another win condition can possibly be achieved by Stealer of Souls + Backfire

Mulligan: try to get Flame Imp, Bloodbound Imp, or Unstable Shadow Blast.

Good luck and have fun on the ladder.