Eldrazi or Phyrexians?
The core concept of the deck is using the sideboard to drastically alter the feel, both from an aesthetic and gameplay perspective.
The first mode has 14 cards that the second mode doesn't: 3x Ornithopter; 2x Retraction Helix; 2x Rona, Herald of Invasion; 1x The Antiquities War; 1x The Lithoform Engine; 1x Master of Waves; 1x Smothering Abomination; 1x Endbringer; 2x Sire of Seven Deaths. This version of the deck is a Rona-centric combo deck with a Merfolk/Eldrazi theme, utilizing Mox Amber + Retraction Helix + Rona, Herald of Invasion
to go infinite, Master of Waves + The Lithoform Engine + Thassa's Oracle to overwhelm with 1/1s and get closer to the blue devotion threshold to win with Thassa's Oracle, Rona, Tolarian Obliterator + Endbringer + Spikefield Hazard + Sire of Seven Deaths using Rona's backside to ping with either Endbringer or Spikefield Hazard to cheat Sire of Seven Deaths into play. Ornithopter + Smothering Abomination + Emry, Lurker of the Loch + Deadly Dispute for generating card advantage with sacrificed Ornithopters that Emry can recur. Overall, the feel of this mode is the corrosive grinding advance of Eldrazi if the initial ritual performed by Rona doesn't secure the game, as 1/1 elementals and Ornithopters feed the growing hunger of Smothering Abomination and it's copies.
The second mode's 14 exclusive cards are: 3x Scrawling Crawler; 1x Omen Hawker; 1x Training Grounds; 1x Pyramid of the Pantheon; 2x Kinnan, Bonder Prodigy; 1x Thassa's Oracle; 1x Harald Unites the Elves; 2x Moritte of the Frost; 1x Sheoldred, the Apocalypse; 1x Soul of New Phyrexia. This version is a Kinnan-centric mid-range deck with a Merfolk/Phyrexian theme that will either deck out with a Thassa's Oracle Harald Unites the Elves + Moritte of the Frost + Moritte of the Frost
or kill the opponent with an army of Scrawling Crawlers overseen by Sheoldred Scrawling Crawler + Scrawling Crawler + Scrawling Crawler + Moritte of the Frost + Moritte of the Frost + Sheoldred, The apocalypse, all achieved through Kinnan's ability to double the mana generation of nonland permanents (primarily Treasures) Kinnan, Bonder Prodigy + Omen Hawker + Omen Hawker + Treasure Vault + Emry, Lurker of the Loch for example generates three treasures every turn by pumping Treasure Vault and saccing it, only to be recurred with Emry. Kinnan's own ability is it's own payoff as well whose cost is reduced by the extra copy of Omen Hawker and Training Grounds, spitting out the necessary pieces of the creature-centric value engines. Overall, the feel of this mode is amassing as much mana as possible as quickly as possible for a grand Phyrexian transformation.
What you might have noticed with both is the role of Emry, Lurker of the Loch. At 3x copies despite being a legendary creature, she serves as the glue for so many of the moving parts. With the ability to enter on the first or second turn due to the number of 0 cmc artifacts in the deck, her ETB synergizes with the Harald Unites the Elves + Moritte of the Frost + Moritte of the Frost
infinite mill combo, providing another way to put Moritte of the Frost in the graveyard (ideally both copies are in the graveyard for that combo). The value of her ETB increases with 3x copies of Essence Flux to flicker her. The recursion of Ornithopter in mode 1 and Treasure Vault in mode 2 adds an extra layer of efficiency when extracting value in earlier turns before the other value pieces can be set up. For these reasons, no matter which version of the deck is being played, an ideal hand is going to consist of 2 of the lowest costing artifacts the deck has, Emry, one to two lands, Mox Amber, and Deadly Dispute or Essence Flux. This is the most explosive start possible, with the option of speed from Deadly Dispute or protection with Essence Flux, especially if putting Emry down is delayed to turn 2 or 3.
Some other cards that warrant a more in-depth explanation would include Mastermind's Acquisition. Due to the adjacency of Mox Amber + Retraction Helix + Rona, Herald of Invasion
to get infinite mana and infinite plays and Emry, Lurker of the Loch + Mox Amber + Mox Amber + Paradox Engine to do the same, I decided to include Paradox Engine in the sideboard with Mastermind's Acquistion in the mainboard, as the extra flexibility offered by a universal tutor paired with it being fetchable and playable itself with Beseech the Mirror makes it a fun one-of. From a flavor perspective, it's also fun rending reality apart and letting either the Eldrazi or Phyrexians peek their head inside of their rival's invasion. Fiend Artisan keeps the deck consistent, providing value to the Training Grounds discount and Omen Hawker's mana generation and preventing the ability for creature cards to be counterspelled (along with a lone copy of Cavern of Souls to reinforce either Merfolk, Eldrazi, or Phyrexians depending on the game). By cheating creatures into play with Kinnan and Fiend Artisan and forcing the opponent to have brought removal, Test of Talents and Grand Abolisher are meant to lock down the opponent's ability to counterplay, with Essence Flux helping keep individual links in the value chains alive.
While this premise is definitely unnecessarily complicated, I was enjoying all of the different play patterns that branch out of Emry, Kinnan, and Rona too much but didn't want to make a deck for each of them and versions including all of them made the decks too cluttered. I also really really enjoy the flavor and feel of the deck and am still playtesting to keep it efficient/competitive for at least the FNM level. Both modes play as effectively Dimir with blue mana being crucial for the starting draw, and mode 2 splashes a small amount of green for Kinnan (which doesn't matter if he's tutored out via Fiend Artisan) and Harald Unites the Elves (which also doesn't matter if it's tutored out via Beseech the Mirror).
I hope you enjoyed reading. Any advice would be appreciated, and I'm especially excited to learn about any cards I missed that work well with both modes of the deck. Some contenders I wasn't able to fit in include Priest of Forgotten Gods; Abyssal Harvester; Springleaf Drum; Kozilek, the Great Distortion; and Jin-Gitaxias, Progress Tyrant to name a few.
