Glen Cook is often called the father of grimdark fantasy. This is his most famous series which most definitely helped grimdark to become the dominant sub-genre of the modern fantasy.
A long time ago even way before J.R.R. Tolkien created his classic trilogy most of the fantasy (or what would later be called such) belonged to so-called heroic fantasy. There was a person, or a group of people who had to perform heroic deeds to save the world most of the time; Lord of the Rings firmly falls into this category. Conan the Barbarian performed his heroics for personal gains. Elric by Michael Moorcock while not being a nice person still saved the world from evil several times.
Welcome to the world of The Black Company. There is no single person in the entire series
who could be called nice by any stretch of the word's definition. You will find plenty of shades of gray though. Their world is dark and violent with almost constant struggle between plain evil and outright evil which would make the Biblical Satan look like an amateur.
The Company is a band of mercenaries with its people just doing their jobs of fighting and dying caught in the power struggle of demigods; at the end of the day even demigods need an army to finalize their conquests; compare it with modern military actions: no matter how many airstrikes you do with even the most modern bombs available only the ground troops ensure the area in question is secured.
The "heroes" of the series are low level grunts of the mercenary band. None of them are nice people, but they do have their own honor code and they try to support the lesser of the evils when they have a choice. The characters of the company are excellent and memorable; they grow on you and you find yourself rooting for them after a while.
Speaking of the characters I cannot help mentioning One-Eye and Goblin. These two are the best frenemies of fantasy in particular and any literature in general. They are minor wizards whose purpose in life is to outdo each other by using any means necessary and as a result they never fail to bring amusing moments.
I am convinced Glen Cook is practically unsurpassed when it comes to the amount of backstabbing and double-crossing in the literature. This makes up for an exciting and unpredictable plot twists.
The book and most of the series is written as the company's annals by its annalist and surgeon aptly named Croaker. For this reason it takes some time to get used to writing style with quite a few people abandoning the series because of this. When you think about it, Croaker really does not have much time for writing fancy literature; unlike real-life Patrick Rothfuss or George R.R. Martin he cannot afford taking his time to write being in the middle of military action.
Glen Cook is a Vietnam War vet and as such when he writes about a war he really knows about it. He never glorifies one; in fact the main idea of the series is that there is no glory in any war, no matter which side you fight on. He is also great when it comes to describing military camaraderie; Croaker often refers to people of the company as family.
So the writing sometimes is choppy and quite minimalist when it comes to the descriptions. If you managed to get past it, you will be rewarded with very good characters and exciting plot. All of the book of the series are hard to put down after the initial confusion and this one is not an exception.
To be completely honest I need to mention I am a big fan of the series, so please take my rating with a grain of salt. It might be half a star lower.