This is an omnibus containing the fourth and fifth books of The Dread Empire series: The Fire in His Hands and With Mercy toward None - respectively. Because the latter book picks up right where the former one leaves it does not make any sense to talk about them separately.
These books make up a prequel to the first trilogy. They show how some alliances which became obvious in the middle of the original trilogy were formed as well as the situation of the major players of the series right before the start of book one - outside of the Dread Empire. This empire which gave the name to the series is actually mentioned exactly once in this book: in the context of somebody's travel references.
I apologize in advance as I cannot give a good plot summary for the fear of giving away big spoilers right from the start; I will have to be intentionally vague. A self-proclaimed Disciple appeared among desert people. He is really convinced about his views and this conviction helps him build initial crowd of supporters. His goal is to unite all of the desert people and some outsiders into a new empire governed by laws that he believes were given to him by his God's angel. He is not very picky about his methods of converting people into his faith and has a very cunning and even more ruthless general to lead his growing army. Does it sound familiar?
The local king went through all the stages people in power usually go in such situations: he ignored the Disciple at first, tried to use brute force methods of dealing with situation including assassination attempts on the latter, then going to a full-blown war - still without full understanding of the current situation.
The writing quality of the series really improves with each book. For this reason I am really not sure about my recommendation on the reading order. My humble opinion would be to go against my usual recommendations and go with chronological order; in this case you would care much more about the people in books 2 and 3 knowing their backgrounds. This is the biggest argument in the support of not reading in publication order.
The counter arguments would be the following: you would spoil the big surprise of the first book and you would have to endure the drop of writing quality while reading the original series. As to the first one: no matter how you look at this, the first book (A Shadow of All Night Falling) is fairly bad and no amount of surprises can save it - I am a big fan of Glen Cook, but I need to be honest with myself in here. The quality of writing in the second and the third book of the series is not much worth than in this book, so it should be a smooth sailing. If I ever reread the series, I would most definitely go with chronological order.
If you expect huge betrayals, double-crossings, grand battles and very complicated politicking typical for Glen Cook you will not be disappointed by this omnibus which gets 4 very solid stars from me.
This review is a copy/paste of my BookLikes one: http://gene.booklikes.com/post/768502/a-prequel-which-most-probably-should-be-read-before-the-first-trilogy