Watchers are among the oldest of races, one of the first to come into existence. Their origins are lost to the hoary mists of time and the Celestials offer no helpful theories. It is suspected, however, that they are more akin to the Celestials than either admits-- possibly descended from the First Gods themselves.
Though they are not truly immortal, Watchers are eternal and ageless. They were one of the first races to develop and created breathtakingly advanced technology, untold aeons before many younger races arrived. The Watchers, much like the Celestials, are capable of channeling the Power Cosmic.
Watchers are bound by a solemn oath of non-interference. This stems from their early involvement with the Maltusians, one of the first intelligent organic races. The Maltusians had a unique affinity for a force called the Emotional Electromagnetic Spectrum. The Watchers, intrigued, assisted Maltus in developing spaceflight and advancing their technology. This resulted in immediate interstellar conflict with the Celestials, the result of which destroyed the First Firmament of the universe and shattered it into infinite multiverses.
Watchers exist at a technological level unsurpassed by any other species. They spend their eternity observing the universe around them in isolation, communing with each other over vast distances only to share information and coordinate research projects.