The ongoing joke with Masters sets is that they release cards into Pauper that ruin the format.
Whether true or not, sets that shift multiple cards to common tend to shake up the metagame. Modern Masters 2017 is the poster child of this as it gave Pauper Augur of Bolas, Burning-Tree Emissary, and Dinrova Horror, three cards that continue to dictate how decks are built.
Double Masters is, at the very least, giving Pauper another such card in Myr Retriever.
Myr Retriever is a powerful card with some serious combo potential, even seeing play in Modern Krark-Clan Ironworks. While Pauper lacks Ironworks it does have Ashnod’s Altar. Two copies of Myr Retriever will give you an effectively infinite loop of casting (and sacrificing) a Retriever. The loop can be paired with Disciple of the Vault, Falkenrath Noble, Hissing Iguanar, or Reckless Fireweaver to kill an opponent outside of combat, Bloodbriar, Glaze Fiend, or Mortician Beetle to win in combat, or Fangren Marauder or Leonin Elder to gain infinite life. Heck, you can put Golem Foundry in the deck to get an arbitrarily large number of Golem tokens. Things are about to get weird y’all.
Pauper is a format that is soft to graveyard based combo decks. This is a byproduct of the cardpool being restricted to commons. While cards like Damping Sphere, Leyline of the Void, or Rest in Peace could completely shut down a Myr Retriever deck such hard stops do not exist in the format. To fight this on the graveyard hate angke the best bet is Faerie Macabre, which is uncounterable. Macabre and its ilk can be fought with Reaping the Graves. That being said the easy counter to Retriever combo is to side in your own “infinite” life engine – Fangren Marauder, Leonin Elder, or Weather the Storm.
What makes Retriever exciting is that every color has something to give to the combo.
White gives you access to Salvage Scout, Sanctum Gargoyle, and Custodi Squire as ways to retrieve pieces that are in the graveyard as well as cards like Prismatic Strands and Dawn Charm to keep yourself alive long enough to combo off.
Blue provides a bevy of options including Impulse, Ponder, and Preordain to stitch things together. Blue also provides cheap interaction in the form of Dispel, Metallic Rebuke, and Spell Pierce to protect the combo, Etherium Sculptor to make everything cheaper, and even the new Archaeomender to, again, regrow dead pieces.
Red can dig deeper with Faithless Looting.
Even Green can get in on the action thanks to Ancient Stirrings. Digging five deep to find two of the key components is huge and can help the deck more consistently assemble its contraption.
All this being said, I would almost certainly start with a Dimir build. Unearth can get back Archaeomender as well as Disciple of the Vault, Dimir also has access to tons of one mana interaction and Transmute cards like Muddle the Mixture, Shred Memory, and Drift of Phantasms. What do these all have in common? They increase the redundancy of your combo.
What can you do to fight these decks? Aside from including your own way to gain life off of dying artifacts, Faerie Macabre and Crypt Incursion both have appeal since they can remove both Myr Retrievers once they are in the yard. Return to Nature gets to use two of its three modes. Cheap artifact removal like Natural State, Fragmentize, and Ancient Grudge also come to mind and already see play. Dust to Dust is a sorcery but can exile two artifacts, while Forsake the Worldly and Shattering Blow are instants that can remove one from the game.
While it might be a fringe deck at the start, I fully expect Retriever combo to be a serious player moving forward. If for no other reason, the deck looks like what a combo deck should look like, which has a significant amount of appeal to a segment of the player base.