Population Migration / Travel
As the elves continued to expand their influence across Archaios and beyond, infighting was an eventuality. One particular sizable coalition of elves held viewpoints differing enough from the majority that earned them exile into the Underdark for their trouble.
They were told to "never again walk in the light of the sun," and so they didn't. Thus, the drow came into being. It is likely the elves that banished them assumed they were enacting a death sentence, but the drow found success in wiping out the vermin that had overrun abandoned dwarven ruins and building new kingdoms atop them, effectively filling in the void the dwarves left centuries prior.