-All the Elder Gods of Earth were spawned 9 Billion (I think?) years ago on Earth by an omnipotent being called the Demiurge Primordial. Their most prominent members were Gaea, Oshtur, Set, and Chthon.
-Oshtur left earth at some point to discover magic and/or explore the higher dimensions of the Universe.
-Chthon himself explored dark magic and became the first Black Magic sorcerer in existence.
-Set killed and ate something to became the world's first murderer, then all the Elder Gods of Earth followed after him by killing and eating each other and other life forms, degenerating into demons in the process.
-Gaea was the one Elder God who didn't degenerate (and Oshtur since she had left), so she called the Demiurge Primordial for help, then mated with him to give birth to the God Eater Demogorge, who slaughtered all the demonic Elder Gods. Demogorge mutated in the process, then when he was done, released all the corrupt energies and flew into the sun.
-While Demogorge was killing Gods, Chthon wrote a tome holding his dark magic knowledge and essence called the Darkhold. He fled to a dimension he made called K'Lay (also called the Flickering Realms), and left the Darkhold behind to maintain influence on earth.
-Seeing Chthon do this, Set left his own influence behind by spawning countless reptilian and demonic races, before fleeing into his dimension the Serpent's Sea. Some other minor Elder Gods of Earth escaped too.
-Oshtur came back, and seeing what happened to Earth and the Elder Gods, she established her own dimension called the Astral Plane close to earth to serve as a conduit for sorcerers and others, and maintain her own influence.
-Gaea decided to establish her own dimension called Avalon, then became the mother Goddess for most pantheons of earth (Gaea for Greek mythos, Izanami for Shintoism, etc.). She also created a powerful magical artifact called the Ebon Rose.
-At some point, Set started siphoning power away from the reptiles of the earth like Dinosaurs. In response to this, Gaea encouraged mammals to replaces reptiles as the dominant species. Enraged, Set absorbed enough energy to manifest on earth and end the mammals, which led to Demogorge showing up immediately. Rather than fleeing this time, Set sprouted several heads and challenged him to a fight. By the end, Set got his ass kicked and banished back to his own dimension, then the dinosaurs went extinct.
-Oshtur shed a tear for a blind boy, and that tear came to life in the form of Agamotto. She raised Agamotto for a bit then left him behind, which led to Agamotto mastering all forms of magic in a spiritual journey to find Oshtur, during which he created the 3 Eyes of Agamotto. He eventually found her, and became the first Sorcerer Supreme. Oshtur also stumbled upon Hoggoth, a Tiger God who was the last survivor of a clan of Many-Angled Ones/Great Old Ones. The spirits of his deceased bretheren were known as "the Hoary Hosts of Hoggoth." Agamotto ascended to become a mystic principality, then joined Oshtur and Hoggoth to form the Vishanti. Oshtur and Agamotto also worked together to forge a powerful magical artifact called the Sword of Bone.
-During the Pre-Cataclysmic age of Earth (before Atlantis sunk), a skeleton wizard called Thulsa Doom started the Darkholders: cults dedicated to collecting pages of the Darkhold and learning Chthon's magic. He eventually got killed by King Kull (a character from the Conan the Barbarian mythos), after which Varnae became his successor, and underwent a ritual to become the first Vampire.
-At the same time, a similar cult dedicated to Set arose, and they created a powerful magical artifact called the Serpent Crown.
-During the Pre-Cataclysmic age, Atlantis was the largest nation, while the second largest was Lemuria. Lemuria was ruled by Deviants, who enslaved humanity en masse and became too populous for the Eternals to deal with, so they called over the Celestials. The Celestials observed the state of earth, and chose to flood everything in an event called the Great Cataclysm. Atlantis sunk, Lemuria and the Deviants collapsed beyond repair, the Serpent crown was lost, and the Darkholders were crippled, their scriptures lost.
-Eventually, a Celtic Sorceress named Morgan Le Fay led a small faction of Darkholders into creating a fully-functioning copy of the Darkhold. They used it to summon Chthon, but the ritual went wrong and Chthon's spirit got trapped in a mountain. Morgan's lover at the time betrayed the cult by comprimising the Darkhold and forcing it to go missing, after which Morgan strangled him to death.
-Chthon experimented returning to earth through a noble man from medieval Europe who held a copy of the Darkhold. That man's wife was infertile, so he gave into the Darkhold's corruption and struck a deal with Chthon to gain a daughter. However, Chthon exploited this to turn their daughter Victoria Montessi into his vessel. Victoria rejected her father's Darkhold cult teachings, and started opposing Darkholders (fun fact, Victoria was Marvel's first lesbian character). Eventually in modern times, she learned her true nature as Chthon's vessel, and different cultists charged after her to ensure Chthon's birth, but Doctor Strange was able to quickly interfere and prevent Chthon's return. Nowadays, I think Victoria's a member of the Midnight Suns or something?
-The High Evolutionary experimented on Romani siblings Wanda and Pietro near Chthon's prison mountain to raise them as artificial mutants. During that time, Chthon's essence transferred to Wanda, and she became his vessel, while he became the source of her Chaos magic.
So pretty much,
4 prominent surviving Elder Gods of Earth: Gaea, Oshtur, Chthon, and Set.
Their 4 Dimensions (The First Four Inner Planes): Avalon, the Astral Plane, the Flickering Realms/K'lay, and the Serpent's Sea.
The Four Cornerstones of Creation (Their 4 Magical Artifacts): The Ebon Rose, the Sword of Bone, the Darkhold, and the Serpent Crown.
Shit, I spent way too much time on this.