Megami Tensei Multiverse

An image of the vast Amala Universe given to the Demi-fiend by the Lady in Black.

The Megami Tensei Multiverse collectively refers to the multiple worlds and universes included in the Megami Tensei series. Nearly all Megami Tensei games heavily feature connections to other worlds and universes which suggests that the games may not occur in the same "universe" but that they still take place in the same over-arching multiverse.

Theories

Digital Devil Story: Warrior of the Demon City

Charles Feed theorized that each great, powerful demon was surrounded by their own "universe" and that only the one could be contacted at once. Thus, upon Loki's death, his universe collapsed and Set's universe took its place, which is why Ohara accidentally contacts Set with the Demon Summoning Program.

Shin Megami Tensei III: Nocturne

The Lady in Black explains to the Demi-fiend that his world is just one of many in the Amala Universe and that the Conception was really the destruction of the old world, which didn't affect the multitude of other universes. There are billions of other worlds experiencing their own Conception and creation of a Vortex World for the sake of creation, each with their own Kagutsuchi. By siding with a Reason, the Demi-fiend is able to choose the rules of the new universe born in its place.

Shin Megami Tensei IV

During the incident 25 years ago the world split into three parallel possibilities as a result of which path - Law, Chaos, or Neutral - Flynn's former life had chosen. Flynn came from the world of Neutral, where the ceiling covers the entirety of Tokyo and its surface became the Eastern Kingdom of Mikado. Chronologically, all three universes of Tokyo visited by Flynn are on the same time spot, as the cataclysm which took place 25 years ago is frequently referred to by residences of all three universes.

Shin Megami Tensei IV: Apocalypse

Stephen sheds a great deal of light on the cosmology of the multiverse. The multiverse is an extension of the Great Will, which is itself the manifestation of all things contained within the multiverse. YHVH was once its avatar, but it grew arrogant enough to declare itself independent from the Great Will and sought to bring the multiverse under its control.

To counteract this, the Great Will brings forth Messiahs- special humans containing a portion of its power that it uses to interact directly with lesser beings, with the intention of having the Messiah thus created break YHVH's hold over his home universe by any means necessary. The multitude of ways that lesser beings have interpreted the nature of the Great Will is due to the fact that it cannot be called sentient in a way humans can understand. Only the most powerful of transcendent beings such as Stephen can come close to comprehending its nature, and even they struggle to do so.

Universes/Dimensions

Digital Devil Story: Megami Tensei

• Assiah
• Atziluth

Megami Tensei II

• Earth
• Expanse

Shin Megami Tensei

• Earth
• Diamond Realm
• River Styx

Shin Megami Tensei II

• Earth
• Makai
• Diamond Realm
• River Styx

Shin Megami Tensei: if...

• Earth
• Makai
  • Land of Nomos
• River Styx

Shin Megami Tensei III: Nocturne

• Earth
• Vortex World
• The various Worlds born of each Reason:
  • Shijima
  • Musubi
  • Yosuga
• Labyrinth of Amala
• Shadow Vortex

Shin Megami Tensei: Strange Journey

• Earth
• Schwarzwelt

Shin Megami Tensei IV

• Earth
• Blasted Tokyo
• Infernal Tokyo
• Expanse
• River Styx

Majin Tensei series

• Earth
• Amnesia
• Paranoia

Devil Summoner series

• Earth
• River Styx
• Alien Dimension
• Extra Dungeon
• Paradigm X
• Corridor of Time

Raidou Kuzunoha series

• Earth
• Dark Realm
  • Alternate Earth of Raido Kuzunoha the XIV
• Akarana Corridor
• Infinite Abyss

Persona series

• Earth
• Maki Sonomura's dimension
  • Avidya World
• Female protagonist's reality
• Nether-realm
• Sea of Souls
• Midnight Channel
• Metaverse
  • Palace
  • Jail
• Theater District
  • Hikari's Theater

Devil Children Red/Black/White Book

• Earth
• Makai
  • Ice Expanse
  • Wind Expanse
  • Earth Expanse
  • Forest Land
  • Fire Expanse
  • Center Expanse
• Deep Hole
• Tenkai / Sanctuary
  • Grass Field
  • Crystal Ring
  • Cosmo City

DemiKids

• Earth
  • Future Rem (Nature/Wasteland)
• Dem
  • Fire Dem
  • Ice Dem
  • Center Dem
  • Underworld Dem
  • Sand Dem
  • Future Dem (Dead Lucifer/Living Lucifer)
• Deep Hole
• Valhalla

Devil Children Fire/Ice Book

• Earth
  • Future/Past Harajuku
• Makai
  • Future/Past Makai
  • Deep Hole
• Niflheim
  • Future/Past Niflheim
• Muspelheim
  • Future/Past Muspelheim
• Ginungagaap
• Tenkai
  • Future/Past Tenkai

Devil Children Messiah Riser

• Valhalla
• Expanse
  • Fire Expanse
  • Ice Expanse
  • Wind Expanse
  • Earth Expanse
  • Center Expanse

Digital Devil Saga series

• Junkyard
• Earth
• The Sun

Devil Survivor

• Earth
• Heaven
• Demon World

Devil Survivor 2

• Earth
• Akasha Stratum
• Providence