Atlantis was the first great superpower of humanity, ruling the ancient world long before Alexander the great or the Roman empire came into play. It was dominant on all continents, leading a technological civilization far more advanced than the 21st century. However, it fell prey to entity that existed beyond our mortal senses:
Narly, the Black Pharoah.
Through sciencey shennigans, it was discovered that Eldritch beings such as Narly existed in other dimensions. This led to three groups with different opinions on what to do about the matter. One wanted to worship them as gods, seeing them as the next stage in evolution. Another wanted to summon and enslave them to serve humanity. The last was conservative and wanted to destroy all trace of them on the mortal plane. This led to a three way civil war which ended with the sinking of Atlantis, as well as a great calamity in the form of a natural disaster that killed billions around the world.
Today this advanced civilization is a myth, as no evidence or trace that it existed remains. Humanity has had to rebuild itself after the calamity, and it has now only begun to reach the stage of where Atlantis was. But you would think that there would be some trace that a superpower like this existed in the ancient world, even after a worldwide natural disaster. As widespread and dominant as it was, there would have been scholars who had wrote about it's history, or some kind of tech would have been found. How could this ancient civilization simply vanish from human history and memory?