The city of Commorragh is established as the primal nodal port within the labyrinthine extradimensional space of the Webway. As it lies outside of standard Eldar jurisdiction, it soon attracts individuals from the Eldar empire who want to avoid attention, particularly those Eldar engaged in the extreme pursuits preferred by the hedonistic cults of excess.
Invention of the Warp-Drive
The Warp-Drive was invented by Mankind sometime in the 18th Millennium. Prior to this, interstellar travel for the voidships of Mankind was limited to sub-light speeds. Travel between star systems was painfully slow, taking many generations, and most human interstellar colonies were isolated outposts established by the crews of Long March generation starships or vessels that made use of suspended animation technology. With the development of Warp-Drive and the Gellar Field, Mankind is soon able to make short Warp jumps over great distances. The Warp-Drive was one of the most revolutionary inventions in human history -- what had once taken many generations of human lives now took only solar days or weeks. Consequently Mankind's colonisation of the Milky Way Galaxy was vastly accelerated in scope and timeframe beyond what would have been possible had humanity been limited to prior conventional reactive propulsion technologies.
The Twilight Cults
Those Eldar leading the new paradigm of total self-indulgence and the pursuit of hedonism rise in status and power within the Eldar empire until they can secede entirely from the Eldar settlements in realspace. They take up permanent residence in the Webway, from which they can plumb the depths of their decadence undisturbed by the puritans and weaklings among their own kind who warn of the dangers of such a path for the future of the Eldar race.