We know that Agamotto was the first sorcerer supreme of earth and discovered how to channel extradimensional energies to fuel programming like formulas (spells) that can bend and slightly alter the physical laws of the universe.
We also know that the Eternals helped develop human society and that Sersi was the inspiration for the Greek Sorceress Circe.
I don’t think this is a coincidence, maybe Agamotto was taught by Sersi to tap into the Energies of the Universe to create certain effects. but Agamotto went a bit farther then just channeling the mundane energies of his own universe and began channeling cosmic energy from other dimensional bodies of existence with completely different physical laws to enhance this new technique he learned, combined with certain formulas he fabricated (the first spells) to create the mystic arts.
What do you think of this theory?