Then must you speak
Of one who loved not wisely, but too well...
--William Shakespeare, "Othello"
The Grigori, also known as the Watchers, were one of Heaven's great tragedies. Created to be teachers and guides to humanity, this giant Choir grew too close to the humans they watched and ultimately sired children by them, including the hideous Nephalim. Dominic, the Archangel of Judgment, put every angel in the Choir on trial and sentenced them all to be Outcast to Earth for an indefinite period. The Grigori are popularly believed to be either extinct, or at least significantly diminished; even speaking their true name aloud is believed by angels to be unlucky.
Some of the Grigori may have Fallen still further to become Skulkers, using their ability to suppress disturbances to hide from everyone and everything. If any Skulkers survive, neither Heaven nor Hell have seen them (despite the best efforts of the latter to recruit them).
Resonance and DissonanceEdit
The Grigori had supernaturally-sharp perception and could hear minute disturbances that might be missed by other celestials. They were the only Choir able to hear the mental disturbance caused when a demon or angel interfered with a mortal's mind in some way. Becuase they were so close to humanity, the Watchers' Roles were especially convincing. That closeness also made them the only celestials able to interbreed naturally with humans, the behavior that led to their downfall. This ability now serves to conceal them from angels and demons alike as they try to blend in with human society.
It was dissonant for a Grigori to ignore a demonic disturbance or one of unknown origin; even if they didn't respond themselves, they had to call for help somehow. Even in exile, they feel compelled to respond, albeit in a far less direct fashion (such as alerting human authorities or attempting to investigate the disturbance on their own).
In their true form, the Watchers looked like humans with a faint aura. However, there was always a sense of solidity about them that marked them as more than human. Their corporeal vessels were literally "head and shoulders" above the rest, tall and fit.
Trial and LegacyEdit
Both Eli and Litheroy are credited with spotting the aberrant behavior of the Watchers ... not in taking human lovers, or even necessarily in having children by them, but in hiding or destroying those children and then continuing to create more of them, a deliberate, selfish choice. Dominic's sentence of exile still stands, but each Servitor is warned not to persecute the Grigori further unless their behavior crosses the line; they were not sentenced to death.
Every so often, a descendent of this celestial crossbreeding will reappear. Some are the monstrous Nephalim, warped in body and soul. But others, known as the Children of the Grigori, seem like normal humans .... save for their natural Symphonic awareness. Those Children that get involved in the War generally drift to the angelic side, leading some to think of them as "Heaven's ace in the hole."