Aurra I - The Fragmented City - Ian Dean - Bog

Aurra takes the "other, fantastical world" genre in a unique direction-by making this other land the place where everyone on Earth is reborn when they die. But it's far from heaven. An acidic and deadly atmosphere is kept at bay by giant lamps, and people are crammed into only a few dozen dense cities governed by an old, seemingly invincible empire.
But Aurrians are also a communal people who can speak telepathically, share stories of past lives, teach the young how to use their ability to control the elements, or even learn the language of animals. Aurra still has the potential to become a great place to spend one half of a full life.
After an entire city of millions stops accepting transferred souls, General Rivia assigns twins Garder and Milla Nolland to investigate. It seems like a simple job at first, but as truth and scale are revealed, they soon find themselves in the company of a young man from Earth, a prince of a hidden kingdom, and an insatiable demon able to traverse the two worlds...