With the addition of the new campaign 'Into the Shadows' I believe mono-shadow finally has a shot at being a competitive deck - if you look back at the decks I made in the past - you'll notice I tried making mono-shadow work in previous sets. I think we finally got there!
So with Into the Shadows comes two great shadow cards in Vara, Vengeance-Seeker which provides the deck a powerful anti-aegis unit so our removal isn't getting two for one'd, a card that provides tremendous value with dark return, as well as provide lifesteal/deadly for pesky aggro or big time decks. Then we have Shakedown which provides great utility of hitting multiple types of permanents that cost 3 or less from our opponents hand, this is big when we want to hit things other than spells like teacher of humility and surprise desert marshal's or scorpion wasps while still hitting those crucial spells from our opponent like Stand Together, Banish, and Annihilate. Nightfall is just a bonus or curse depending on your view.
Last but not least we have Sealed Writ which is a colorless power source early but a card that later can be a way to finish off a creature, relic weapon, or opponent and since we don't have a card over 7 or multiple faction requirements this is just a free-roll in my opinion.
I know I have Eremot, the Gathering Dark in the market and it does have sweet synergy with Dreamsnatcher but it hasn't been amazing unfortunately though Im still gonna keep it in there for now as it does provide an evasive threat and win condition against decks with very few units. I think a good replacement might be The Last Word if you wanted a different market card that could win those long grindy games.
So what got better for this deck outside those new cards
Dreamsnatcher - to me this card went way up with the release of into the shadows. It synergizes with our discard effects and is an effective 2 drop against aggressive decks as well as providing a late game effect which can outright win the game.
Keredon Merchant - Well with the release of Fall of Argentport we got a powerful tool to help us add consistency and specific cards we want to exchange for particular matchups.
Shadowlands Tyrant - Its finally time for this unit to shine - with having another must answer creature the opponent must deal with and in conjunction with discard and dark return - he truly can take over a game all by himself.
Cull the deck - a cheaper and leaner scheme - this card filterer is a welcome addition
Devastating Setback - A power modal spell which protects our powerful units or stops swarm/aggro decks from overwhelming us but also providing a +1 strength to our dreamsnatcher
Your good matchups: Combrei Aggro, Skycrag Aggro, Rakano Valkyries, Stonescar Aggro or Mid, Praxis Tokens, and any heavy time based deck.
Your medium matchups: Temporal, any Mask of the Torment deck, and Armory.
10/11/2018 Update: So with the nerf to temporal control we can cut Eremot as its the weakest card by far - so with that I added another In Cold Blood to the market as there will likely be a pick up in justice decks but this spot I think could be alot of things like Umbren Reaper, Sleepless Night, Shadowlands Feaster, Vara, Fate Touched, The Last Word, Touch the Umbren, Slimespitter Slug, Stray into Shadow, or Thief's Pick.