http://www.pcgamer.com/assassins-creed-origins-trailer-reveals-the-birth-of-the-brotherhood/

Assassin's Creed: Origins is, as the title so subtly hints, a game about the origins of the Assassins. In case there was any doubt, the new trailer makes it very clear that this is in fact what's going on. Bayek, you see, was the last of the Medjay—an "elite police force" of ancient Egypt, as the Wiki explains—but that just wasn't enough to get the job done. 

"There is a new creed now," Bayek intones as the trailer reaches its climax. "From darkness, we have come. And in darkness, we shall stay. We are Medjay no longer. We shall be known as Assassins."