The dust jacket proclaims, "The year is 3589, fifteen hundred years after Commonwealth forces barely staved off human extinction in a war against the alien Prime". The arithmetic is flawed ("Judas Unchained" was circa 2380, so about 1200 years have elapsed), but Hamilton's story-telling, character development, and world-building are virtually flawless.
After a prologue introducing Inigo and the mysterious Void, which is being studied by the ancient Raiel race, Commonwealth humans, and a number of alien races at Centurion Station near the galactic center, the story alternates between the main time sequence following various Commonwealth characters, and chapters detailing Inigo's past "dreams" of apparently medieval humans. As the story unfolds, it is soon revealed that Inigo's "dreams", which have been recorded and shared with millions of other humans through the gaiafield (a kind of telepathic network enhancement that many have chosen for themselves), are believed to originate from the Void and to chronicle actual life inside the Void. Millions of humans believe life inside the Void to be idyllic -- a religion, the Living Dream, grows around this belief, culminating in a plan for a mass Pilgrimage into the Void.
In conflict with the Living Dreamers are the Raiel, at least one faction of the human-created non-physical intelligence ANA, and an alien race called the Ocisen Empire. The Raiel have been struggling against the Void for a million years, having seen the Void expand and devour countless inhabited worlds, and they expect that a Pilgrimage could trigger another deadly expansion. The Living Dream Pilgrimage, if not stopped, may cause the destruction of the galaxy. But the Living Dream religion persists, and ANA:Governance, controller of the immensely powerful Navy (a military force which has grown in power since defeating the Prime in "Judas Unchained"), seems inclined to protect the Living Dreamers from outside influence or attack.
A number of characters are introduced, both in Inigo's dream chapters, and in the Commonwealth chapters. Aaron is searching for Inigo (who has disappeared sometime after having his famous dreams), but he does not know why or who sent him, and he seems strangely accepting of the mission script he is following as it is gradually revealed to him. Edeard is an orphan boy from Inigo's dream who is trying to find his place in a world that is both hostile to him and able to be shaped by his telepathic powers. In the Commonwealth where technology can be employed to keep one's body fit and perfect, Troblum is a physicist who is extremely fat, a collector of memorabilia from the Starflyer war, and gifted at creating advanced technology and weapons for whoever is willing to feed his obsessions. Araminta is a young, ambitious first-life woman who is trying to start a property-development business herself after a divorce, but her adventures are just beginning.
Justine, Paula, and Oscar from the Commonwealth Saga also have storylines, and there are appearances by several other characters from the Commonwealth Saga. Commonwealth Saga characters notably absent from "The Dreaming Void" are the alien Silfen, any escaped alien Prime, the SI (sentient intelligence), the Bose motile, and Nigel and Ozzie (although Ozzie has apparently become a common swear word).
There is a timeline at the end of the book that provides a number of helpful historical references to fill in some of the events between the end of the Commonwealth Saga and the events in "The Dreaming Void". For example, "2833--Completion of ANA first stage on Earth; Grand Family members begin memory download into ANA rather than to SI....3001--Ozzie produces uniform neural entanglement effect known as the gaiafield." There are no significant spoilers in the timeline, and since Hamilton does not fill in all the details in the narrative, it may be helpful to read the timeline before beginning the novel.
While there are at least 9 viewpoints running simultaneously in "The Dreaming Void", I did not find myself bored by any of them. In contrast, I frequently skipped over various uninteresting viewpoints in Hamilton's Nightsdawn trilogy. Hamilton has done an excellent job of choosing the viewpoint characters in "The Dreaming Void", balancing the action between them, and keeping all of them relevant and interesting. I appreciate that Hamilton has not spent many words summarizing events from the Commonwealth Saga, since that would merely bore readers who are familiar with those events, and probably not help those who have never read those books. Also, since this is not strictly a sequel (1200 years have passed and the most important characters are new), I suspect that it can stand on its own for someone who has not read the previous books. However, as expected in the first book of a trilogy, "The Dreaming Void" does leave quite a few plot lines hanging at the end. If I had not already read the Commonwealth Saga books, I am sure I would feel compelled to read them while waiting for the next Void Trilogy book to be released.