This city was created by a Chaosborn named Annavilla. This person was a genderless being born from a metal created by the chaotic void.
The chaotic void had several such places where extremely rare and valuable materials could be found. But only the truly powerful or a handful of decently powerful people being protected by someone truly powerful were able to mine and collect them.
That was one of the reason why Zeta's ability was seen as a game changer. Not just for making ordinary people to travel from universe to universe but also for supplying a huge number of workers for a mine like this.
The Chaosborn who had created this city was born in one such chaos mine and was taken to an universe by a different Chaosborn who was unable of recognise it as a living being at that time.
