A quick re-cap – not too spoilery if you haven’t read the series.
Thrown out of heaven for seducing human women, Semyaza and two hundred other angels spent thousands of years being tormented in hell for their crimes. When they finally broke out a hundred and forty years ago, they did the same thing again. And then they mysteriously disappeared.
The Rephaim are their offspring. Half-angel and half-human, they were tracked down and raised by Nathaniel, a fallen angel seeking redemption. He believes the Rephaim’s destiny is to find their fathers and deliver them back to the Angelic Garrison.
The Angelic Garrison
An army of archangels. The Fallen served with them before disgracing themselves. It was the Garrison who sent Semyaza and the Two Hundred to hell.
Gatekeeper demons are also tracking the Fallen since their escape from hell, and are looking to wipe out the Rephaim along the way.