Their society had begun to decay. The diversity of genetic evolution was failing them. In their desperation they assumed stewardship of their own biological destiny. They became their own makers not out of natural adaptation but as a result of their own terrible will. They were wretched and immortal. They were dire and full of fury, at creation, which had abandoned them and at nature, which despised their existence.
They worshipped gods that didn't exist so in their madness they created them.
The Cradle of Leng was the city of their birth. Leng was grown from the genetic remnants of a billion years of experimentation. It was alive, this city, and tortured. It was written on the walls of Leng that torture and pain give depth and strength to its foundation.
When the city was old and near it's own death they made way to give it rebirth through itself. They inseminated the city with the seeds of malformed gods of their own creation, filled with eons of existence and the seething chaos of the thoughts of long dead divinities. The city was tortured yet again with miscarriage and abortion time and time again until one day the seed held fast and began to grow.