The source of the Dark Side

In Star Wars: Aftermath, which was created specifically to give canonical pre-text to TFA, Palpatine's advisor was asked what the Empire should do to continue Palpatine's legacy. And he said this:

“No Sith remain,” Tashu says. “And the lone Jedi that exists—the son of Anakin Skywalker —possesses an untouchable soul. At least for now. We must instead move toward the dark side. Palpatine felt that the universe beyond the edges of our maps was where his power came from. Over the many years he, with our aid, sent men and women beyond known space. They built labs and communication stations on distant moons, asteroids, out there in the wilds. We must follow them. Retreat from the galaxy. Go out beyond the veil of stars. We must seek the source of the dark side like a man looking for a wellspring of water.”

About 50 pages after, we're introduced to a group called the Acolytes of the Beyond. They are a small group dark side worshipers, obsessed with finding Sith artifacts -- namely Vader's saber. Put two and two together, and it's obvious that they must be well aware of the Empire's efforts to find the "source of the dark side". Their name affirms this. "Acolytes" means a ministry or a group of followers. And "Beyond"? Well, it would make a whole lot of sense if it's the same "beyond" that the Emporer's advisor was talking about -- "Beyond the veil of stars".

So how does this all tie together? I believe that after the fall of the Empire, the Acolytes of the Beyond were banded together with the task of finding the source of the dark side and everything assimilated with it. Since "the son of Anakin Skywalker possesses an untouchable soul.", what would be their next best option? The grandson of Anakin. They, with the help of Snoke, seduced him and made him their leader, as well as recruiting others and ultimately changing their name to the Knights of Ren.

After they successfully seduced Kylo, it was a race between them and Luke to find the source of the dark side, a.k.a the First Jedi temple. It would make sense if that is where the first Jedi were seduced and where the first Sith/the Dark Side were created. I also think that Luke is there to protect something - an artifact of some type - to keep it out of the hands of Snoke/the First Order and prevent them from becoming incredibly powerful.

Up until TFA, the Knights of Ren were trying to find the source of the Dark Side.

A place that Palpatine felt was "beyond the edges of our maps".

What were Kylo/Snoke obsessed with finding throughout the entire movie? A piece of a map that the Empire didn't have. A map that not only leads directly to Luke Skywalker, but also the very source of the Dark Side itself.