Billions of years ago, just after the creation of Earth which was still in its cooling process, the incarnation of the power of Creation known as the Demiurge seeded the Earth with its ethereal energies which led to the birth of the Elder Gods. First among them were Gaea, Chthon and Set. The Elder Gods proliferated and ruled the Earth for untold billions of years.
One of them, the Goddess Oshtur, had a fascination with what lies beyond the stars and decided to leave Earth and travel across the universe. She was later known as the Bright Lady of the Dawn and Goddess of Stars.
The evil Chthon sought out to study the ambient mystical energies of the cosmos and thus became the first master of the black arts. He learned all about what was and what will be, what could be and what shouldn't be. Chthon experimented on lesser beings, twisting their essence and playing with their lives. He also inscribed all his arcane knowledge in a indestructible parchment later known as the Darkhold.
The kind Goddess Gaea watched over the evolving life in the Earth's oceans and destined herself as the protector of all life on the planet.
The serpent God Set soon spawned two monstrosities which would aid him in his conquest of the planet. Set soon discovered that he can absorbed the essence of his sibling by devouring them, and adding their powers to his own. Thus he set out a rampage of murders of other Elder Gods.
Soon after, all the Elder Gods followed Set's footsteps and participated in the mass murder of Elder Gods and they slowly degenerate from Gods to demons, all except one, Gaea.
In fear of the danger the other Elder Gods have on Earth's growing life, Gaea called out for the Demiurge and together they spawned the first of the new Gods, Atum, who possess the power of the Sun itself.
Atum started to slay the Elder Gods except for Chthon and Set. Having the gift of foresight, Chthon has come prepared and casted a spell that allowed him to escape to another dimension. Set saw what his brother has done and copied him, allowing the serpent God to escape as well..
Since then, Gaea is the only Elder God allowed to exist on Earth. She also infused the Godly essence on Earth, thus becoming part of her. She also spawned the next generations of Gods. Gaea still lives on Earth where she is known as Mother Nature and the Mother Goddess.
Upon Oshtur's return to Earth, she has learned about the horrific fate of her siblings, so she decided to commit self-exile in order to keep her sister Gaea safe and the spell of the Demogorge intact. Oshtur soon joined with two other principalities, Hoggoth and her son Agamotto and formed the eternal Vishanti.
Each of the four remaining Elder Gods created a realm near to the Earth Dimension.
Oshtur created the Astral Plane, a realm which can be shaped by the minds of its populace and visitors.
Gaea created the realm known as Avalon, a dimension of lush and verdant forest and filled with magical beings such as faeries, elves and trolls. (However Separate from the home of the Celtic Gods, of the same name)
Set created the dimension known as the Serpent's Sea, an abyss of tears and cold blood that ripples with Set's insatiable hunger, filled with serpents and monsters.
Chthon created the dimension known as the Flickering Realm, a realm which is constantly changing and the only sure thing about it is danger as demons dwell everywhere in this dimension.
The influence of the Elder Gods continue to this day. Gaea survives and is known in every mythology as Mother Earth, Set's influence the mutliverse via the Serpent Crown and still worshiped in some cultures. Oshtur fights with her Vishanti to protect the universe from mystic threats. Chthon still dwells in the shadows, spreading evil throughout history via the Darkhold and his chosen mortal host, Wanda Maximoff.
Some writers consider the mystical beings known as the Gibborim to be Elder Gods, but their origins are still shrouded in mystery to confirm the theory.
The Elder Gods individually possesses vast mystical and cosmic powers. Their power is far greater than that of any current gods and deities of Earth and beyond. They each possess power that dwarfs that of any Godheads and Hell-Lords, and a minuscule fraction of their might greatly dwarfs that of even the most skilled and powerful sorcerers.
Each of the Elder Gods, even though only spawned on Earth, possesses sufficient power to affect and influence things on a near-Mutliversal scale. Set is said to be the source of the Serpent Crown and all its counterparts on alternate realities, Oshtur as one of the Vishanti is a member of the greatest trinity of mystical beings in the Multiverse, Gaea is revealed to be the fountainhead of all life in the Multiverse.