Forging a key

A long time ago, there aren't any barriers between the worlds. People could get in and out, some aware, but others don't. Some had houses in this timeline and their backyards were on the other. Some timelines had abundant materials, and some were barren. But it's not a big deal. But humans discovered that some other world could produce extraordinary things, like plants that turned into gold or immortality pool. Then some men became greedy and hurt, even learned to kill another. It started in a single murder, the famous one in the bible.

And then two, then eventually became a war. Men that won the brighter and abundant place become stronger and don't die at all. More and more people were trying to get into that side, which would kill as many people. The immortal side prevented greedy people from coming from their place. Sometimes they do charity and share their blessing through miracles. They made protective barriers.