According to the intro, V1's latest firmware update was installed on August 6th of 2112.
V1's default arm is called "Feedbacker", after a Boris album.
The arm acquired from V2 in 1-4 is called "Knuckleblaster".
Since it fires a drill spear, the Screwdriver Railcannon is a reference to Tengen Toppa Gurren Lagann. To cement the reference, the HUD icon features a core drill, and killing with the drill grants the style bonus of +GIGA DRILL BREAK.
"DevilMayQuake.com" redirects to the game's Steam store page.
The game launches with a splash screen disclaimer reading "This game contains scenes of explicit violence and gore", identical to the one Capcom used in the late 90's and early 2000's for M-rated games like Devil May Cry and Resident Evil.
The style meter is nearly identical to the one used in DMC, down to the letter-graded Idiosyncratic Combo Levels. It also rewards experimentation, combos, and switching your weapons up.
There's a secret Dual Boss in Limbo consisting of two Swordsmachines named Agony and Tundra, suspiciously similar to Agni and Rudra of Devil May Cry 3: Dante's Awakening. There's another secret Dual Boss in Heresy that operates on the same premise, consisting of two Insurrectionists named Angry and Rude.
The entirety of the boss fight with Gabriel is almost definitely a shout-out to Vergil, primarily his boss fights in Devil May Cry 3, with very similar outfit color palettes, movesets, and mid-match taunting, right down to activating a new form reminiscent of Devil Trigger half-way through the battle.
Three references to Swans: the armless husk enemies are called "Filth" (and their eyeless, huge-mouthed faces even resemble the cover of the album in question; this reference was confirmed by Hakita's notes in the developer museum), the name of stage 1-2 is "The Burning World", and 4-2 is titled "God Damn the Sun".
Swordsmachine's design is, according to the developer museum, inspired by Blame!.
Cancer Mouse can be fought as a hidden boss in one of the levels. Similarly, the hidden area he's found in is a replica of the Dungeon from earlier videos on Civvie 11's channel.
Punching the coin used for the Marksman variant of the revolver grants the FISTFUL OF DOLLAR style bonus.
The Stalkers draw visual inspiration from the series of the same name, being gas mask-wearing mutants.
The bosses of P-1, Flesh Prison and Minos Prime, are directly inspired by Legion and Nuculais from Castlevania: Curse of Darkness, confirmed in this tweet. The former is a giant, slow flesh monster that houses the latter, a glowing humanoid with a visible heart.
The entire Tomb of Kings area in 4-3 is just a massive chain of references. To wit:
The unique blue skull in Stage 4-3 wears a pink beanie cap which is based on the avatar of Matt McMuscles. For another Two Best Friends Play reference, the opposite corner of the room has Woolie Versus and Pat Stares At as skeletons inside of a coffin (fittingly they share the coffin with Pat being a small skeleton).
You can ride a rocket over to the distant lighthouse island in stage 5-2. There, you can find a pair of skeletons that, when approached, provide the caption, "Hark! Neptune has struck them dead."
5-S features a shark that strongly resembles IKEA's stuffed shark plush BLÅHAJ, a reference cemented by the lore stating that it "originates from Sweden" and is a "sign of femininity" (referencing the plush's association with the transgender community). It also has the Frog (?) from Iron Lung, using the same model as the creature from that game and only showing up in a bloody body of liquid, also like its inspiration. Also featured in this level are the Chomper from DUSK and the Metal Fish from the Forges episode in AMID EVIL.
In 5-4, The Leviathan's heart resembles a human screaming in agony, complete with a pair of hands clutched on its face. To bolster the reference, the heart is always screaming.
The bonus track included in the ARG, "UltraChurch", has a segment starting around 36 seconds in that is made to resemble a part of Doom II's first level theme, "Running From Evil", only played on an organ and with a different key.
During the rematch with Gabriel in 6-2, one of his new taunts is “I’ll show you divine justice!” This is the quote used by Tales of Symphonia’s Lloyd Irving, when he uses his Falcon’s Crest Mystic Arte in English dubbed versions of his games. Additionally, Gabriel is wielding a blue and a red katana in the rematch, which is similar to the blue and red swords Lloyd must wield to use Falcon’s Crest.
Sisyphus Prime'sAnti-Nihilist personality is based on The Myth of Sisyphus by Albert Camus, with his terminal entry directly quoting the book.
The boss of 7-1, The Minotaur, is a reference to House of Leaves by Mark Z. Danielewski. Just like the Minotaur in House of Leaves, the Minotaur in the game has both its name and backstory crossed out whenever it is mentioned, and is associated with an endless, impossibly dark expanse that it dwells in.
The Gutterman's death sound is taken from Giant Robo, albeit with filters and edits applied.
The Earthmover is named in reference to the Have a Nice Life song of the same name, which is fittingly about an army of world-destroying artificial beings.
A book in 7-4 refers to the Final War becoming such that "War no longer needed its ultimate practicioner". This is a reference to Blood Meridian calling war "the ultimate trade", with man as "its ultimate practitioner".
The Soap and Crystal of Madness cheat menu items are both references to identical items from DUSK
The red Rocket Launcher variation is called Firestarter.
The Jackhammer/Alternate Shotgun you can find in 7-2 is essentially the Impact Hammer from Unreal Tournament given the ULTRAKILL treatment: A pneumatic, jackhammer-like melee weapon that can be charged up for a powerful blow and can also be used for push-jumping off terrain and even deflecting rockets if you have good timing.
Levels
The Sandbox level is a recreation of gm_construct.
Stage 1-3 is titled "Halls of Sacred Remains", titled after the similarly-named "House of Sacred Remains" area from Castlevania: Lament of Innocence. The very Castlevania-esque music used for this level is even called "Castle Vein".
Stage 2-S is named "All-Imperfect Love Song", after the very long eponymous track by World's End Girlfriend.
In addition, the stage's gameplay revolves around a character within a dating simulator who is well aware her entire existence is fake, and as a result has been overcome with existential dread, an obvious homage to Doki Doki Literature Club!.
The ride leading to the fight with the Corpse of King Minos is a reference to Half-Life's level "On A Rail": in both games, you get to ride a tram with very clunky controls to manage its speed.
The landmines put down by Guttertanks and Big John are very similar in function and appearance to Half-Life 2's Hopper Mines.
Stage 4-1 is named "Slaves to Power" which is both a reference to the game and the Iron Maiden album. It's the first level of the Greed layer that has heavy Egyptian aesthetics, which further highlights the connections.
Stage 4-1's theme song is called Dune Eternal. The song itself contains a reference to "Into Sandy's City", one of the better-known tracks from the soundtrack of Doom II.
Stage 4-2 is named "God Damn the Sun", another reference to Swans.
Stage 4-S is named "Clash of the Brandicoot". It's basically a Crash Bandicoot level using Ultrakill's Greed layer visuals, complete with V1 mimicking Crash's run animation, spin attack and his victory dance if they manage to break all the crates.
Stage 5-1 is named "In the Wake of Poseidon" after the second album from King Crimson.
Stage 5-2 is named "Waves of the Starless Sea", which is both a reference to "Waves of the Random Sea", by post rock band Natural Snow Buildings and to "Starless" by King Crimson.
Stage P-2, "Wait of the World", is titled after a Kayo Dot song of the same name.
Stage 7-1 is named "Garden of Forking Paths", after a short story by Jorge Luis Borges. Its second background music is titled "The World Looks Red", the name of a Sonic Youth song whose lyrics were penned by Michael Gira of Swans.
The theme that plays when you fight the Minotaur is named "Bull of Hell", with it being a riff on Bull of Heaven.
Stage 7-3 is named "No Sound, No Memory", after the Merzbow song "I'm Coming to the Garden... No Sound, No Memory". Its second background music is titled "Danse Macabre", after the art genre of the same name.
7-S is named "Hell Bath No Fury", a play on the phrase "Hell hath no fury like a woman scorned.", a misquoted passage from The Mourning Bride. Meanwhile, the level itself is a Whole-Plot Reference to PowerWash Simulator in terms of directly lifting its mechanics, but the concept of cleaning up after the mess of a First-Person Shooter hero also makes it an allusion to Viscera Cleanup Detail. However, activating a secret within the level turns off the lights and summons ghostly Drones which can be only be defeated by being sucked into a vacuum, turning it from Power Wash Simulator to Luigi's Mansion.
The former is a giant, slow flesh monster that houses the latter, a glowing humanoid with a visible heart.