Pact of the Sand and the Tree

There is a mysterious Shine emitted by The Tree. It speeds up the growth of the plants, acts as a day/night indicator, and most importantly, it keeps many monsters at bay. There are, however, some creatures, which can slip through. Usually some smaller beings or beings so hostile or incapable of feeling pain, that they don't care about the deterrent properties of the Shine.
Almost all of the Beasts from Above can resist its powers. Because of that, a deal has been made between the People of the Surface and people living on the lower layers of The Pit. A deal, a pact, a contract. Between two sides that could survive on their own, but can prosper when cooperating.

The details of the deal

There are a few points written down in the contract that are more important for ensuring better conditions for both sides. First, it is the duty of the People of the Surface to get rid of any Beast that gets down into The Pit (at least as much as it is humanly possible) or keeping in contact with the lower layers so they can be alarmed of the incoming monsters and deploy the suitable countermeasures.
In return, the inhabitants of the lower layers supply the people above them with any resources needed to survive on the endless sands that can't be acquired up there.
There are many more paragraphs like that, ensuring mutual benefits for all who signed (or were represented by those who did) the contract.

Type

Treaty, Diplomatic

Location

The Surface

First changes

The first result of the pact was the creation of the transit settlements, cities, and villages where the inhabitants of both sides can easily trade with each other without moving farther into the environments they are not accustomed to.