What does this mean? Agency means a character's ability to affect the outcome of the narrative. The idea that the protagonist of the story should have agency is a core tenet, albeit often an unstated one, of writing fiction. The conventional logic goes that whatever the stakes of the story, it must be the hero's actions that determine the outcome. If the story is about an intergalactic war, it has to be the hero who blows up the bad guy's space station, not anybody else. If the story is about a kid lost in the wilderness, it has to be the kid who finds a radio and calls for help - he can't just be rescued by someone who accidentally stumbled across him. The same rules apply even if it's a tragedy and the hero doesn't succeed: Hamlet, Oedipus and Tony Montana all bring their fate upon themselves, through their own actions. They have agency.
But in The Black Company, the protagonists have practically zero agency. The story concerns a war between two groups of enormously powerful wizards, each with vast armies and territories at their command. But the protagonists are not the wizards but the Black Company, a band of mercenaries who are nothing more than a single pawn on the chessboard. Through the whole book they are used and abused in a byzantine web of politics and betrayal. The narrator, Croaker, bears witness to several key events, but he is always on the sidelines, watching wizards kill and double-cross other wizards. (Yeah, the novel is pretty big on wizards, can you tell?)
Croaker does make one decision at the end of the book that seems like it will lead into him and the Black Company having more agency in the next two books of the trilogy. But taken as a stand-alone work, it's quite unique. Yes, there have been plenty of works of literary fiction that scuttle the agency of the protagonist - Thomas Pynchon's Bleeding Edge and David Mitchell's number9dream come to mind - but rarely has it been done in fantasy fiction, and without losing the page-turning quality of a good adventure story.
There is often an assumption with fantasy fiction that it should feel empowering to the reader: that you put yourself in the shoes of the mighty hero, and feel a sense of triumph when they succeed. The Black Company evokes a feeling not of triumph but of helplessness, more akin to the tone of Infinite Jest or The Trial than your typical fantasy epic. Yet on the other hand, the book still cracks along at a great pace because we still get to see, through Croaker's eyes, a lot of dramatic events. The sensation when you turn the page is not of pushing on excitedly, but of being carried along inexorably towards a dreadful conclusion.
