I read Hammerfall when it first came out in hardback. Our Hero is Marak Trin Tain, a warrior who finds himself in a mass of people being herded to “the holy city.” He is one of a group of people who have been infected by a peculiar madness that “the Ila” the immortal ruler of the holy city finds interesting. Marak Trin Tain is also someone the Ila finds interesting, and despite the fact that Marak is the son of one of her enemies she puts him in charge of an expedition to discover the source of the madness.
What appears at first to be fantasy turns into science fiction when it is revealed that this particular form of madness is actually a message from a team who is trying to contact the Ila. It turns out that the Ila was part of a group guilty of attempting to use nanotechnology to terraform worlds. The nanotechnology did not work very well, and destroyed the worlds in question. (The Ila’s own world is another world that ended up having an ecological disaster as a result of terraforming with the technology in question.) Unfortunately, one of the worlds damaged by the terraforming attempts was the homeworld of aliens called the ondat. The ondat have plans to destroy the world and the nanotechnology the Ila still has possession of via bollides.
The people who have contacted the Ila want her to evacuate to a place of safety. (Note: This place of safety is going to be on planet because no one wants the Ila’s nanotech cooties.) They have made what they hope is a deal with the ondat in exchange for finding a way to repair the damage caused by the nanotechnology. (The ondat are of the “mostly mutually incomprehensible” variety of aliens. It is very hard to make deals or arrangements with beings that are mutually incomprehensible.) Marak of course does not understand much of this except for the parts where he has to get as many people as possible to the refuge before rocks fall and everyone dies, and there are people in the sky who are very, very unhappy with the Ila. (The latter of course is something he can sympathize with.)
Hammerfall has some epic worldbuilding and Cherryh’s trademark complex character interactions and motivations. It’s a really great book, but I like the sequel Forge of Heaven a little better.