Fire World is one of the farthest planets from the Earth. While we do not have a detailed description of the planet, we are told that it is populated by a dominant species known as the Fire-Tasters, and it is the Fire-Tasters who act as Hosts for the Souls. The Fire-Tasters main form of sustenance comes by eating the smoke of a burning Walking Flower.
This planet has caused a considerable moral dilemma for the Souls. While Souls have no compunction about taking over a Host’s body, believing that they improve the Host’s welfare, they do not want to cause harm to any living thing, especially sentient or quasi-sentient life forms. After populating the planet, it was discovered that the Walking Flowers were, themselves, sentient or quasi-sentient. Thus, living as Fire-Tasters automatically made the Souls within them murderers.
The Souls hope to use science to discover another, less brutal, form of sustenance for the Fire-Tasters. They have even imported Spiders to the world in hopes of discovering a solution. Once another form of food can be located for the Fire-Tasters, the Souls can assimilate them without committing murder. The Walking Flowers can then be assimilated as well.
Wanderer states that the reason the planet was not abandoned is because the Souls have a pressing need for Hosts, and because many Souls wish to remain there. However, the planet does have a very high relocation percentage.
Burns Living Flowers, the Soul who accompanies the other group of humans at the end of The Host, is from Fire World.
Because of the moral issues associated with the planet, Wanderer chose not to live there.