Basically, the Watchers are the Illuminati. I go a bit more in depth in this video, but essentially there was a group of humans called the Illuminati, but they seemed to follow the ideology of these fallen angels. The eye symbol can be traced back through history to various pantheons which themselves fit the methodology and description of the Watchers.
[Video: https://m.youtube.com/watch?v=n1AC6BEqeBk]