It's Book 3 of the Night's Dawn Trilogy. It was for fun, and kinda was for a bit... but sooooo much word vomit! Each book averages about 1100 pages, and, well, sometimes you can burn out.
So the trilogy essentially takes place in a fairly extensive galactic Confederacy with all sorts of planets with different cultures and economies. On a back-water planet, a portal to "hell" is accidentally opened causing the spirits of the dead to come through and possess the living. These Possessed, once they take over a planet, take the planet out of the universe so they do not have to worry about dying again. The Confederacy is trying to stop them while also dealing with the terrible widespread depression the discovery of an afterlife entails.
There's also trade deals, terrorism, magic, Satanists, Al Capone back from the dead trying to create a galactic empire, lots of badly written sex, and some aliens. And a sort of nuclear arms race as well.
It's big, it's somewhat unwieldy, which means some of the more pointless side stories are more frustrating... sheesh, some of the main ones are frustrating! (I mean, Al Capone? Really? There's only so much suspension my disbelief can take, Hamilton!)