From the back of the book:
Angels Are Everywhere
They are under the everyday world, behind it, beyond it. They are sent by the God-Machine to enact its will through time and space, delivering messages, building infrastructure, protecting some people, killing others. You were one of these angels...
But Not Anymore
Now you are one of the Unchained, a fallen angel who defected to the human race. Yours is a world of false identities and clockwork conspiracies, stolen faces and hidden works of the Machine. You cannot - will not - return to the sterile embrace of Heaven.
All You Can Do Is Reign In Hell
Demon: The Descent includes:
Rules for playing a fallen angel in the World of Darkness.
Powers suiting the former servants of the God-Machine, from reality-hacking Embeds to earth-shaking demonic forms.
Details on the God-Machine's angels, as will as stigmatics, people altered by exposure to its mysteries.
An overview of the activities of both the Machine and the Unchained in the city of Seattle, where even time is not what it seems.
Requires the World of Darkness core rulebook. Includes the rules updates from The God-Machine Chronicle.