WMG / Digital Devil Saga

The world of DDS is a sequel of sorts to Shin Megami Tensei III: Nocturne
Hitoshura/The Demi-Fiend sided with Yosuga, and produced a world where the strong devour the weak. This explains the cannibalism, the presence of the Demi-Fiend, and, to an extension, why the goal of the game is to euthanise Brahma, who is this world's manifestation of the Great Will, the end of the second game was a more positive "Conception."

The world of DDS is another world within the Amala Network
For starters, The Lady In Black did mention that the Earth that Hitoshura was born in, was one of many in the Amala network. The spinning barrels atop of the heads of Brahma's many forms are Amala Drums, but are used for a different purpose within the DDS world. God's power could be explained as him using Brahma as another avatar of his because: the way that the world was deleted is like what happened in Nocturne, but much stronger and God works in mysterious ways in BOTH series. Plus the Hitoshura as a Bonus Boss could be interpreted as Lucifer trying to recruit more demons in a different world, asking his most trusted General to go scout the worlds.

The Demi-Fiend is so powerful that you only won because he wasn't going anywhere close to all-out
The Demi-Fiend is definitely one of the toughest, if not THE toughest, of bosses in a series already known for being Nintendo Hard. Yet if you've played Shin Megami Tensei III: Nocturne, where he was the protagonist, the attacks he uses here (the Mute-causing Javelin Rain, Heat Wave, Stun-inflicting Xeros-Beat, and the overkilling Gaea Rage) were attacks that were fairly mediocre, as well as the demons he calls. The only demon that could probably do well in Nocturne's endgame is Titania (though the Megidolaon-casting Pixie is certainly nothing to sneeze at). Yet Gaea Rage does SEVERAL THOUSAND damage when your HP cap is 999, and that assumes your stats are near their max. Now imagine he was actually TRYING. Enter Metatron, Shiva, Beelzebub, Red Rider, Daishoujou and DANTE. Meanwhile he'd be throwing Focus'd Spiral Vipers and Freikugels all over the place. Not to mention he'd be using the Masakodos magatama which nulls EVERYTHING EXCEPT ALMIGHTY.
  • While I do agree with this, there are two things you seem to have forgotten. One of them is that he probably was using masakados, as the only attacks that work against him are almighty attacks or attacks with types that weren't in his game, and therefore he couldn't block. Also, there is a ridiculously powerful pixie in Nocturne like the one he uses in the bonus boss fight if you keep your pixie from the very beginning of the game all the way to the endgame
    • Perhaps he got overconfident/let his pride get the better of him, explaining why he didn't have every powerful skill/demon he used in Nocturne?
    • Or he's leveling up those demons as fusion fodder.
  • Pfft. Gaea Rage still does mega damage in Shin Megami Tensei III: Nocturne, but in that game, it was just a glorified physical skill that has a larger HP cost than Deadly Fury. Plus, even though he was only involved in a Japan-only remake of Nocturne, Raidou Kuzunoha would also be summoned if Demi-Fiend was actually going in full-power, along with the Trumpeter, Pale Rider, Black Frost, and Mother Harlot.

The Tribhvana are the humans that Jinana, Mick and Bat were based off of
. Especially with the latter, because there is that one member who (like Bat) is utterly batshit insane.
  • Probably not. Practically everyone in the Junkyard is a dead human, and the Tribhvana are still alive. The exceptions are people Sera knew: Serph, Heat, Cielo, and Argilla. Even if she made more people, she probably didn't meet them. No one would put a two year old girl/valuable test subject in the same room as a team of insane soldiers.

The Four Angels and Meganada were members of the World Stabilisation Committee that tested the Demon Virus on themselves

Greg's Junkyard Avatar (Lupa) was deliberately introduced into the simulation.
  • Think about it. Before entering, he was one of the foremost enemies of the Karma Society. This means he was at least very well versed in military tactics. Once he died, the scientists of the Society took whatever data they could from the corpse and poured it into the Junkyard, intending for their enemy to become one of their soldiers. Fits what a Karma Society scientist would do.
This game is a very distant sequel to Megaman Starforce
Instead of EM Wave Changing people bonded so close to E Mians that human thoughts became part of the data networks. Atma Brands are like little hyper compartmentalized Star Carriers/Hunter V Gs. Persona is a world where the EM beings are instead manifested externally.

No one knows how to swim
The Waterways does have the underwater levels, but the Junkyard is a computer program. The fights take places just as they do on land, even though the Embryon should have their movement impaired by the water. It's not just lazy real-life programmers; that really is how they fight in-game. The people who changed Sera's program couldn't simulate water properly, and figured that it wouldn't be used in urban warfare. Sera couldn't design water because she never learned to swim.

The Demi Fiend in the game isn't the real Demi Fiend
Given that the Junkyard is a computer simulation, and Atlus existed in-universe (they made the Shmup minigame), perhaps the Demi Fiend is actually someone's Nocturne game data inserted into the Junkyard. The fight seems like a hallucination because the game data doesn't match up with the rest of the world. The fight was put in either by a bored programmer, or as another test to see how the Asuras would fare against overwhelming odds.

Sera's song only works in the Junkyard
Even though Madam Cuvier's plan revolves around her song, she's never shown testing it in the real world. Sera's song works in the Junkyard because she created it and can control its residents to a limited extent (she forces Mick the Slug to let her go, the Karma Temple guards let her pass, etc.), and the song is a way of focusing her powers.