Hearthpwn

This will happen, Questlock has enough removal and heal to stall before, but it lacks of draw to finish the quest in time and lacks the finisher to end the game. So, last expansion, they just die before the late game. In this expansion, we got Free Admission to tutor minions (hopefully fish the Questing Explorer to boost your draw, but getting other minions is not bad either since it combo pieces or removal).

Here's the guide to do the otk.

1. Finish the quest.
2. Finish 3 C'thun Spell, and keep 1 in hand.
3. In the same turn, play last C'thun Spell, play Tour Guide, Polkelt then Hero Power. You will get 0 mana C'thun in hand.
4. Play the C'thun next turn after you clear their board (you can use twisting nether, double cascading disaster, topdeck kelidan, or any other removal in your hand because your deck are full of them). That 30 damage should go right to their face!

Interesting things worth to mention here are:
1.  The quest should be completed fast enough because now, after your turn 2 hero power, you have the Free admission on turn 3 which could lead you into drawing Questing Explorer and/or Safety Inspector (safety inspector is better since it doesn't crowded your hand). This opens the high possibility to draw total 9 cards at the start of turn 5. So, If you managed to have plot twist on that turn, you basically finished your quest at the following turn. Crazy right!
2. The additional plot twist is better used to find that C'thun pieces faster since that was your win condition. And Vargoth can be used as more C'thun pieces as well if you desperate to end the game faster (it should worked).
3. In the last C'thun Spell+Polkelt turn and you draw 0 mana C'thun, you can just drop it if your your C'thun Spell actually clear your opponent board. Also, you dont have to keep Tour Guide for that last C'thun Spell+Polkelt turn if you already have 0 mana Polkelt or 0 mana C'thun Spell. As well as you dont really need 0 mana C'thun if you have 0 mana Twisting Nether (Assuming you dont play against heavy deathrattle deck).

Have fun with this otk!