Mulligans
Aggro Mulligans
Kazakus is good for a swing turn vs token and zoo. Doomsayer should be played asap on turn 2.
Control Mulligans
The HP from DK Bloodreaver will create increased value and tempo the earlier it is activated, therefor keeping DK Bloodreaver in opening hand is good vs control. Mistress is a good turn 3 heal after double tap. Kazakus is always great to get out early, and corrupting mist is good too shut down priest shenanigans and other developments. Rest is just good early game cards.
Deck includes some demons to make Krul the Unshackled and Bloodreaver Gul'dan more playable. Having the ability to remove one of your opponents minions with Doomguard after playing a 9/10 mana card gives you much more survivability. Voidwalker andDespicable Dreadlord can help some too. Kabal Trafficker is a bit experimental, and could possibliy be changed if it does not deliver.
The deck has 4 main win conditions;
Instaplay a huge board your opponent using Krul the Unshackled
Outvalue and tempo your opponent with Lord Jaraxxus
Develop a free huge board and an unbeatable hero power with Bloodreaver Gul'dan
Outvalue your opponent with all the decks value tools like Elise the Trailblazer, Kazakus and Kabal Trafficker on top of the allready mentioned cards above.