Somewhat.
IMC beholders - which the party haven't encountered yet - are actually mortals who experimented on themselves to increase their mental and magical faculties. The mind flayers are a subsidiary servant race who did most of the mutatin/technical stuff (like the cloners from SWII: AotC). The mind flayers escaped from their beholder masters and now use the technology to build their own empire. Most of the mortal races remember little of the beholders, but remember being ruled by their servant races - illithids, yuan-ti and sahuagin. Most frightening of all is the fact that the eye-tyrants are not dead and don't like being forgotten by history...
I realise that the above is more history than myth so how about this.
In the Dawn Age came the original mortal race - no one remembers their name or what they were truly like, except that they were powerful beyond mortal ken and were unafraid, even of the gods. They spread out across the planets which orbited their sun, which they called the Hearth of Worlds, because by its warmth all life grew. They lived in an age when the boundaries of the universe were more fluid.
The Origines (there is an acute over the e) - as theologians call them - developed all manner of skills and powers, not the least of which was the ability to grow and change themselves. They separated their society into castes, with the most powerful ruling and the least powerful serving. Scholars are divided over whether this caused discontent or whether each was happy in his role.
The Origines began to encode their caste system into the worlds around them and even into their own bodies. Animals were bred to serve each caste and each caste was in turn bred to serve its function. This is how the mortal races began - being created from the star-born lifestuff of the Orignes.
At the bottom were servants, each bred to suit their world or function. From these were born the elves, the dwarves, sahuagin, suited to their respective worlds. Gnomes and halflings were bred to service the devices and the little spaces of worlds. Humans were the multi-role workforce - adept at learning and changing from generation to generation.
Above the servants were the masters of lore and craft, superior to the lower caste because of the knowledge which they possessed. In this caste grew the illithids, masters of the biological magic which underpinned the entire of Origines culture. Some scholars argue that there were others, such as the now far fallen descendants of Gith. These scholars claim that the githyanki were the craftmasters of the Origines and that the githzerai were their researchers of mystical secrets.
At the very top were the rulers, about whom almost nothing was known, only that, over time, they became immortal beings of pure intellect and magic.
No one knows how long this situation continued, but all agree that the achievements of the Origines were without par. However, at some time the Origines empire had grown to fill the entire of the solar system and they began to look to the Void beyond for further frontiers to explore and conquer.
However, in the Void the Origines found powers unlike anything they had ever before encountered and these powers had a dark purpose. Offering powerful secrets in exchange for allegiance, the Demonic powers of the Void seduced the Origines. By an insideous conspiracy, the Demons and Devils who were drawn into the worlds by the Origines caused the inhabitants of the worlds to darken the sun, for Demons from the Void cannot abide the light and warmth from the Hearth of Worlds. By dint of a gargantuan effort which stretched even the powers of the Origines, the first sun was extinguished.
What resulted is a time of terror and almost limtless corruption which is known by some as the Age of the Void Fallen, and by others as only the Darkened Age. With the power of the sun gone the Demons and Devils of the Void invaded the worlds en mass and fractured the empire of the Origines. The Origines were split into various factions and made to war for the entertainment of their demonic masters. New races were born in this time, such as the corrupted orcs, the ogres and giants. For an unmeasurable time, hell stalked all the worlds.
However, not only Demons and Devils dwell in the Void, and the spirits of the stars, the Celestials, saw the extinguishing of the Hearth of Worlds and they mourned its passing. Then they gathered in force and led an army to overthrow the darker powers and restore the sun. Their war is said to have lasted for over a millenia and by its end all peoples had sworn themselves to one side or the other.
The war between the celestials and the dark powers ended when the celestials gathered all of the dead from the endless conflict of the Darkened Age and, purifying and consuming them in an almighty ignition, created a New Sun. Because it was born from the bodies of so many dead, it was called the Pyre, though some still knew it by the name of the old sun, the Hearth of Worlds. Caught close in the ignition of the New Sun, the world of Ashen was burned, scoured of almost all life and of its past, so that only ruins remained.
The ignition of the Pyre drove the main of the demonic forces back into the Void. Many of their servitors, such as the now corrupted Yuan-ti, were left behind. Others - like the mind flayers - were taken as slaves into the Void, only to return at a later time. The mortal allies of the celestials all agreed upon a compact and the final power of the New Sun was to purge the racial memories, so that the follies of the Dawn Age and of the Origines would never be repeated. Recognising this power of the New Sun, many of the remnant demonic servitors hid from the New Sun, to preserve their Dawn Age knowledge. This was the beginning of the split between the elves and the drow, as well as the permanent sea life of the sahuagin. That is why, although the greatest evil comes from the Void, it is the deepest darks of the world which hide evil.
The celestials returned to the Void to become stars, to keep watch for souls which might become lost in the Void.
Of course many theologians debunk all this as silly myth. After all, how can this story account for the gods? And what of the other great beings. And if the New Sun drives away the knowledge and lore of the Dawn Age, how is it the Mind Flayers have returned. Nonetheless, this is the most comprehensive creation myth of all the worlds which orbit the New Sun, the Pyre, the Hearth of Worlds.
