Demon World

The Demon World is the main setting of The Promised Neverland. The Demon World is part of the world that many years ago, was split into two between humans and demons after The Promise was signed with the Ratri family. The part of the world belonging to the demons is comprised of many bizarre ecosystems and creatures, some of which include feral/bestial demons.

History
Millennia ago, before the construction of plantations, both demons and humans shared the same world, which led to frequent hunting of humans by the demons. Some humans soon after did the same, while the rest worshipped them out of fear of getting eaten. This became a continuous cycle as both sides killed and feared one another. After countless violent wars and massacres, a clan of humans called the Ratri family stood up to the demons and made a treaty known as The Promise.Citation needed

Bayon's Proposal
Bayon and the government of demons held a meeting, discussing how they should consume orphans when they are forbidden to do so after the Ratri clan's proposal of a treaty. Bayon came out with the solution of building the farms to "cultivate" the young of the humans so as to satisfy the demons' need of consuming human flesh, as well as to comply with The Promise. Therefore, Bayon's solution was accepted and included as part of the treaty.

The humans gave some of their children to the demons as a parting gift when they fled from the demon world. The children were later kept in "plantations" (institutions that take care of the children) as livestock for the demons to consume.

The Signing of the Treaty – The Promise
We shall make a treaty as follows: "Human beings shan't hunt 'demons' anymore. In return, 'demons' shan't hunt humans either. This world we live in shall be divided between both.".A proposal —otherwise known as The Promise— was formed by the first head of the Ratri family. It declared that "humans and demons are restricted from further hunting." Afterwards, the world was broken into two — one side belonging to the demons while the other belonging to the humans.

Birth of a Religion: Heathen
Some demons did not approve of this deal and thus refused to even nibble any of the children born in the farms. This caused these demons to be branded as "heathens" and were shunned for their behavior. However, while their religion prohibits them from eating humans born in the farms, they wait for The Promise to be broken and be able to hunt humans again.Citation needed

Demon Forest
A vast area of dense trees and wildlife located outside the human plantations, it contains a variety of bizarre fauna and plants, including the Bloodsucker Trees Area, where sentient vines drop creatures beneath the surface and suck the nutrients out of them.

Heathens Hideout
The demon Sonju created a path comprised of dead Bloodsucker Trees in order to serve as a hideout for himself and his companion Mujika. This is to avoid the plantation demons who reject them as "heathens" for not eating humans. However, it is later revealed that the heathens aren't forbidden from eating humans, but from eating humans born in the farms. As such, they wait for the day humans born in the wild will come to pass so they can hunt again.

Grace Field House
One of four farms where the human children left behind by their ancestors are raised as livestock by former livestock known as "Mamas" and are branded with tattoos and shipped out at certain ages to become offerings to an unknown demon lord. It was set on fire by the children from ages 5-12 who escaped to the demon world after discovering its true purpose. After having burned down to the ground, the children were moved over to a different part of the plantation under a new mama.

Glory Bell
A separate plantation where livestock tattoos are determined by a combination of letters and numbers. A group of kids escaped from there and were journeying to Minerva's second shelter at Goldy Pond before they were captured by Poachers and all killed, other than Mister and Lucas.

Grand Valley
Another premium plantation farm where children are being raised to serve as livestock for the demons. In this farm, children are being prepared to be shipped out to Goldy Pond because Grand Duke Leuvis serves as a purveyor so he has a lot of power both in Goldy Pond and Grand Valley. All members of the Goldy Pond Resistance movement and the likes of Theo, Monica, and Jake came from this orphanage.

Factory Farms
Factory Farms are hundreds of farms which contain cheap meat for the demon population. The children in these farms have no free will of their own or any power of speech, as they are only kept barely alive and spend all of their lives being fattened up by demons to be eaten. One of the children living in Goldy Pond, Adam came from a Factory Farm.

Shelter B06-32
Shelter B06-32 was Minerva's first hideout and was set up as both an armory, a source of knowledge about the world's affairs, and a safe haven for escapees who possessed promised pens and were smart enough to escape the plantations.

Goldy Pond
While Shelter B06-32 was only a temporary hideout for escapees to meet with Minerva and regroup, Goldy Pond was his actual base of operations. It contained a small village that served as a front for the true entrance, hidden under a silo and several ladders underground lies a floating island, which contains an elevator to the human world when a promised pen is inserted into it.

Trivia

 * The demon world is theoretically divided into four areas, as per Minerva's pen, with Grace Field House as the origin point. Going clockwise, they are labeled A, B, C, and D.