Main concept: put cards in graveyard and use them later meanwhile casting 6/6 flying, trample demons.

Enablers:

4 Stitcher's Supplier put 3-6 cards in graveyard.

2 Glowspore Shaman put 3 cards in graveyard and help with hitting land drops (and can chump block the Steel Leaf Champion).

4 Jadelight Ranger explore 2 times and put cards in graveyard, help getting lands (and can kill Steel Leaf Champion).

4 Doom Whisperer surveil 2 which puts cards to graveyard and helps to get a control over your draws. Extremely powerful if board state is stalemate. 

Payoffs:

3 Lazav, the Multifarious becomes a creature from your graveyard, eg.:

• Stitcher's Supplier for 1 mana if Lazav is going to die -> put more cards to graveyard.
• Midnight Reaper for 3 mana if another creature will die -> draw a card.
• Demon of Catastrophes for 4 mana -> 6/6 flying trample without downside.
• Doom Whisperer for 5 mana -> 6/6 flying trample with ability to surveil.
• Muldrotha, the Gravetide for 6 mana -> cast other cards from graveyard.

2 Journey to Eternity // Atzal, Cave of Eternity bring back creatures from graveyard.

1 Muldrotha, the Gravetide casts the cards that you've previously put to graveyard.

Other synergies:

You can sac Stitcher's Supplier to Demon of Catastrophes -> utilize useless body to get a 6/6 flying trample and put 3 more cards to graveyard.

It is also cool to sac creature enchanted with Journey to Eternity // Atzal, Cave of Eternity to Demon of Catastrophes -> get a 6/6 flying trample, transform the journey and start getting your creatures back. Possibly profit from ETB trigger on saced creature (like Jadelight Ranger, Stitcher's Supplier, Glowspore Shaman).

If things go bad you're able to transform Arguel's Blood Fast // Temple of Aclazotz and sac your fatties to get some life points while you still can cast them again later.

We would probably go with 26 lands, but having 2 Glowspore Shaman and 4 Jadelight Ranger should ease hitting the land drops.

Mana base:

We would probably go with 26 lands, but having 2 Glowspore Shaman and 4 Jadelight Ranger should ease hitting the land drops.

The only 2 cards with U in mana cost are Lazav, the Multifarious and Muldrotha, the Gravetide. Lazav is a 2-drop so it is important to have to blue mana turn 2. We have 12 lands that produce blue mana, including 4 shock lands and 8 check lands. Probability of having at least one blue mana source on turn 2 is roughly 85% on play and 89% on the draw (assuming you didn't have to mulligan and not considering if mana comes untapped). There are more blue cards in the side board so that's another reason we have 12 lands that tap for blue mana.

Sideboard options:

• Carnage Tyrant will be good vs other creature heavy decks as well as vs control because it is not counterable and has hexproof. This lil dyno provides value even if you bin it - Lazav, the Multifarious may become a 7/6 tramlpe hexproof dude.
• Assassin's Trophy may help with destroying artifacts/enchantments/Teferi, Hero of Dominaria.
• Cast Down has low enough mana cost to hold opponent's early creatures.
• Duress is a classic card vs control.
• Price of Fame might be especially good if opponent has enough legendary cards.
• Ritual of Soot is incredible vs token/aggro/go wide decks.
• Sinister Sabotage might surprise you control opponent (and surveils).
• Thought Erasure punishes slower decks.
• Unmoored Ego is going to totally counter combo decks (like turbo fog) and also beats Teferi, Hero of Dominaria.

Perfect curve out:

T1 Stitcher's Supplier -> T2 Glowspore Shaman / Lazav, the Multifarious -> T3 Journey to Eternity // Atzal, Cave of Eternity / Jadelight Ranger -> T4 Demon of Catastrophes -> T5 Doom Whisperer -> T6 Muldrotha, the Gravetide / Vraska, Relic Seeker.

Bad situations:

 – T3 Goblin Chainwhirler kills Stitcher's Supplier and Glowspore Shaman.

 – Knight of Grace is hard to block with our 1-2-3 drops and it has hexproof.

 – Vine Mare punishes if you don't have an appropriate blocker. Luckily, we have 4 Jadelight Ranger that block both Vine Mare and Steel Leaf Champion.

 – Spells that exile creatures are hard to beat. We are happy to trade creatures since that generates us a virtual card advantage (or not virtual if we have Midnight Reaper on the battlefield) when we return them from graveyard, but exiling our creatures with cards like Settle the Wreckage, Vraska's Contempt and Seal Away hits us hard because it turns Lazav into semi-useless card.

– Speaking of exiling things, exiling cards from out graveyard also hurts us. But it's not the end of the world if we have cards to stack the graveyard up again. Sentinel Totem is an example of such a card, as well as Deadeye Tracker.

– Demon of Catastrophes might be a dead weight in our hand if we don't have other creatures on the battlefield or if don't want to sac the ones we have (Lazav, the Multifarious, Midnight Reaper, Muldrotha, the Gravetide). Best card to draw in topdeck mode on an empty board are probably Doom Whisperer and Vraska, Relic Seeker but unfortunately the worst would be Demon of Catastrophes.

– Aggro decks might be tough to beat. Maybe deck needs more Cast Downs main deck to kill threats like T2 Steel Leaf Champion or Benalish Marshal or Tajic, Legion's Edge because using Assassin's Trophy early in the game is just going to help your opponents to ramp. Main thing we need to do is to sideboard Ritual of Soot and hope to draw it, thankfully we can filter out our draws with our surveil cards. Although, Ritual of Soot hits us as well: approx. 65% of our creatures cost 3 mana and lower, so if you board in the sweeper - board some spot removals / hand disruption spells as well so you don't kill your own dudes.

Other possible considerations:

Deadeye Tracker of our own might be better than Stitcher's Supplier in late game but definitely worse in early game.

Charnel Troll seems to be a good card if the deck needs to be more aggressive since it might become a 6/6 trample on turn 4.

Steel Leaf Champion is just good but it is hard to cast it turn 3. Nevertheless, it's a good target for Lazav.

Nullhide Ferox might be a good replacement for carnage tyrant (especially vs Grixis Nicol Bolas, the Ravager // Nicol Bolas, the Arisen.

Vine Mare of our own punishes opponent's control deck.

Bone Dragon seems like it has to be in this deck: we fill up our graveyard pretty fast (potentially up to 11 cards by turn 4 if draw goes like Stitcher's Supplier into Glowspore Shaman into Jadelight Ranger binning both explore non-land cards into graveyard). So bone dragon becomes as an almost always available creature to cast for 5 mana on turn 5. However, its stats are worse than either of our demons. Also, exiling 7 cards from graveyard to cast bone dragon works against plans of Lazav, the Multifarious, Muldrotha, the Gravetide and Journey to Eternity // Atzal, Cave of Eternity. Need some play tests to know for sure.

 The Eldest Reborn is another awesome card that could fit well in this deck. I'd consider replacing one or two of Doom Whisperer with The Eldest Reborn depending on how playable the demon would be in upcoming standard format.