Hearthpwn

I like the fact that Zombie Chow can work as 3rd and 4th Mind Blast (just imagine turn 6 - Chow, Chow, Auchenai, Circle)... 

It is as always ... just use frost mage to win agains any boss :-D 
honestly I actually liked Loatheb because of that Alexstrazsa deny ... 

Sorry but I believe that some cards in this deck are just not so good. For example what is your reason to have Sunwalker? Patchwerk destroys it by his hero power and you have nothing left, Sludge Belcher is just plain better for this matchup. Also 2 pyroblasts seems like a bit overkill since you want to end this boss battle earlier than turn 10 ... 

This is by far the funnest deck I've played so far.  Just had a game against a miracle rogue where I dropped Voidcaller turn 4, he then eviscerated it to bring out my Jaraxxus which I then faceless manipulated into a second Jaraxxus the next turn.  He then tried his miracle shenanigans but I played defender of argus right after, so I had 2 taunted up Jaraxxuses (Jaraxxi?) and he couldn't do a thing against them.  Smacked his auctioneer with one Jaraxxus then slapped his face with the other.  It was glorious :D

I had been thinking about trying to come up with a decent Voidcaller/Demon deck, and happened to get a Warlock quest today. I basically never net-deck, but decided to try this one on a whim. Holy **** this deck is fun to play. My first decent decks in Hearthstone were basically HandLocks, and while this is quite different, the experience with that play style comes in very handy.

I have to say, Voidcaller deathrattle -> Jaraxxus + Defender of Argus is now my favorite thing in the game. I can only imagine what it must be like to be the opposing player when this play goes down. My guess would be slightly comical at first (because losing your Jaraxxus due to Voidcaller has to be a bad thing, right?) and then terrifying. Especially if they are a Priest. What the hell are they going to do with a 4/15 with taunt? Turns out, usually nothing, followed by losing quickly.

I don't have the Cabal Shadow Priest's what can I replace them with?

You can definitely tweak this deck to suit the meta you are running into.  I prefer Ysera, the extra cards at the end can really help and generally a better target for healing.

He didn't run loatheb

I think this deck is better at higher ranks. Up to rank 10, any rush deck generally works. Switch things up and see what works for you.

Which card did he replace for Loatheb?

Mind Blast is better than Leeroy.