The book was a work of fiction, fantasy with some magic and politics.
The plot followed some sort of disaster that destroyed a large piece of land/city/nation and the way it changed the landscape of nearby countries/peoples/organizations in the vacuum. Not quiiite a medieval level fantasy, as it had less of a kingdoms, castles and knights feel, but it certainly was set in a pre-industrial age world.
We had multiple character 3rd person POV dealing with the fallout. Low fantasy perhaps, I don't remember there being any non-human characters. What magic I remember seemed to be religion based. There was something about a birdcage with a metal mechanical bird in it? The big disaster was not a natural event, something triggered it. The book started with an encampment of some warlord on the outskirts of whatever place blows up.
The main POV characters I remember were:
There was an older man who had been either the head of a holy order or very highly positioned at some point previously, but was now living in anonymity. He saved the life of a young man around the disaster, near the encampment, and the two of them had a mentor/devoted student relationship. The older man did something to prevent the younger man from rising to power in a new holy order, trying to protect him somehow?
There was a woman spy who worked on behalf of her father and was undercover in the encampment. She had tons of siblings, and most, if not all of them, had been trained similarly in subterfuge and were planted all over the world. Her father was a puppet master/information broker, steering the course of world events from behind the scenes. He referred to his children by number and gender (my 47th daughter, and so on). She started the series doing spy stuff and was then sent to join a group of horse rider people (to spy there too perhaps?). She ends up breaking ties with her father after becoming involved with one of the horse rider leaders.
The horse rider leader man was in charge of a faction/smaller part of the horse rider people, but may have ended up in charge of multiple of smaller companies/bands. I think they were skirmishers? He was in line to be a big cheese and had to take up the mantle. The horses were a big part of their culture, but they did have a settled hometown that they returned to. This horselord guy had a twin brother that died when they were small children. At some point it's revealed the puppet-master father was behind poisoning the young twin brother.
Physically, I read this in paperback format. The cover had mounted riders moving across tall grass/fields/plains with the title against a black bar. The title was something to do with calamity, catastrophe, disaster, apocalypse, annihilation (not Jeff VanderMeer's Annihilation, this much I do know) and may have been a single word.
The book was in English. I found it secondhand about 7-10 years ago. It was an average length novel meant for adult readers, not YA nor children's lit. I suspect it was published in the 90s or later. This novel was either a standalone or the first in a series
Appreciate any thoughts on what this book could have been!