So what's the story behind the Pthumerians?

So I just wanted to understand their significance in the overall story of the game, but I never understoood why that name kept showing up.

Like why those people are so important to the game's world itself, and if the player was supposed to meet one as I don't know if they went extinct at the start of the game.

Like if they're precursors or something as I really want to know more about their overall purpose in the game itself.