Hearthpwn

Can Zola be replaced by Zavrothar? Or would it need to be brewmaster?

Is Zilliax necessary? I see it a lot recently; a lot more than when it was new, somehow. Should I craft it? Any replacements?

And 2 voidcallers in a reno deck? You sure?

Zavrothar has antisynergy with this deck due to it being a demon. I wouldn't use it, pulling it with voidcaller instead of the other demons would be quite bad.

Zilliax is just a very good control card in general but it doesn't have much synergy here, totally replaceable IMO, there is already lots of healing. 

Voidcaller is the key card to the deck, it enables all the demon combo plays here and two should rarely hurt you.

edit: played 10 games, lost 2.

One loss was to a greedy control priest who had great rolls while my RNG was terrible. Despite chalking that loss up to bad luck, I still felt like this deck might have trouble with control priest in general, but didn't face anymore to see for sure.

The other loss was to a fast quest shaman. Even though I had a great curve to trade early, I got screwed and died midgame to a  wide board of dorks and a lava rager or some crap by not being able to play kazakus or zephrys for AOE due to not having drawn either voidcaller. Didn't make me cut a copy but I definitely thought about it.

Ultimately my card changes were -1 healbot, -1 zillax, -1 dirty rat, +1 carnivorous cube, +1 al'ar, +1 gnomeferatu.  I have bad history with rat backfiring and gnome has more highroll + ragequit potential IMO so that is reason for that switch.

I didn't miss healbot or zillax, since I didn't face many aggro or burst decks to need them but the cube never really performed so I'd cut it if I played anymore (maybe add hellfire?). 

Al'ar turned out to be a really good addition and won several games alone or in combo with expired merchant (surprisingly).

Yeah, I tried Zavrothar once in my own version, one that has no voidcallers but 2 voidlords. Fewer demons meant Sense Demons was more likely to draw the duplicate sooner, which I found useful. 

For some math: without playing sense demons, your chance of drawing one of your only duplicate cards reaches 51% on your 9th draw, up to 75% when you have 15 cards left in your deck; 91% with 9 cards left, and 97% with 5 cards left.

Also, what ranks and region were those games? Because I managed to climb from gold 8 to diamond 8 using a fast quest shaman in just a few hours, in EU

I haven't tried it in this list because I don't care much for Voidlord as a card, power creep has badly weakened it since its time in the limelight. IMO Enhanced Dreadlord is so much better in the same role and I have loved it in this deck.  I would also really hate how Voidlord would dilute Karenthrad Ebonlocke and Bloodreaver Guldan's battlecries with 1/3 Voidwalkers (and choke the board with N'zoth also), one of which I usually end up copying to grind out wins in long control games.

After a few awkward games with double early Voidcallers, I did end up swapping out the duplicate for a Twisted Knowledge. I've found that I am often reaching winning board states in the mid/early-late game based around combos with Cube, Al'ar, Zola, Umbra, or Sylvannas, so the duplicate Voidcaller didn't really feel needed as I have win conditions other than a big board of demons with my deck alterations.

I still think 2 copies of Voidcaller is fine though, as long as your meta isn't hyperfast.  As you pointed out the chance to draw one duplicate copy before you usually need or want to play highlander cards is quite high.

I've only played 5 more games but had no more losses (13-2 now) and twisted Knowledge is performing well as an options engine.  The only cards underperforming for me atm are Bran and Doomsayer. Doomsayer is just too hard to trigger or use effectively without a freeze combo (mage main here haha) and having cut Dirty Rat from the deck doesn't help with that either. Bran is of course, on paper, one of the best cards in the game but in practice, with this deck, it always ends up burning my cards due to hand overflow or just acting as a 'win more' card that I didn't really need to play. I may cut Bran to re-add Rat to the list.

I play on NA in low platinum atm, don't really have time to climb, though this or a similar deck can definitely do it if you don't mind long games.  Been farming control and spell mages (30ish % of matchups) with this deck in my meta situation mostly.