I'd meant to read this quite some time ago but iBooks had some glitch and claimed the file wouldn't open and it took entirely too long for me to remember that I needed to follow up on the problem in a place and time I could track down the glitch (in iBooks, not the file). I finished the main River of Souls trilogy a year ago and in an odd way, having that much of a gap before reading "Nocturnall" worked very well, because we return to Ilse and Raul a considerable time after the end of Allegiance.
"Nocturnall" works more as an extended character sketch than an independent story. I wouldn't advise reading it if you haven't read the rest of the River of Souls series. But if you've done so, it's a...well, I don't know that I can call it a "pleasant" conclusion, but a fitting one. It's an end of life story that gives closure, in the bittersweet way that end-of-life stories must. Without giving any spoilers, we see the pair in their prime, settled into ruling the land and having raised two generations of desendents. It isn't a perfect life, but a satisfying one. And then we see the past come around full circle to return the consequences of past deeds. In the aftermath of that, the story reminds us of the vivid "realness" of the eponymous river of souls--the flow of identities across many lifetimes and ages--that gives the inhabitants of this world a rather different relationship with death and with eternity than our own relationship. That relationship turns what might otherwise be tragedy into...something else.
The story flows smoothly and--though the ending is entirely predictable from a knowledge of the setting--that only means one can sit back and enjoy the language and imagery without too much angst about what will happen next.