It's from the last days of the older phase of Precursor civilization (not that they called themselves that; they called themselves makers or life makers), and attempts to summarize the hora quan war in an easy-to-follow way. It's a tertiary work whose position on the tradeoff curve between telling a straightforward easy-to-understand story and giving all the evidence is far toward the former end and it openly admits this. It is also intended to be possible to contain only on low-tech non-eco-powered media like scrolls or stone tablets, which is unusual for a book from their civilization at the time.
They were working on a terraforming project and removed some dark eco from a planet to make it nicer. They sent it away, aiming for it to end up in orbit around a foreign sun where it could stay for ages. This was far enough before the invention of time travel that the time period in question was inaccessible by the time anyone double-checked where it had ended up. And it appeared that the dark eco asteroid was missing, and that a planet around that sun had an unusual amount of dark eco and signs of a recent impact. And that planet had people living on it. Not just life, but intelligent life. Intelligent life that had faced some manner of apocalypse, given the ruins, but was now rebuilding and reaching for the stars. They had already visited the other planet in their system. And they were cruel, destructive, violent, and dark.
There's an interlude about their ecology and speculation about what it might have been like before it was warped into something so evil. Then the narrative resumes.
The aliens, whom the makers named the hora quan, were warring with each other, even though you'd expect that they'd get along best with each other since they were the same species. The amount of war, and the extent to which it was open and not happening through proxies or cold wars or space races, was frankly surprising given both their tech level and the fact that they were still around to keep doing it.
There was one group in particular that had recently destroyed or conquered some of its neighbors and was in the process of trying to conquer a much smaller polity. This aggressive group was particularly disliked even by their conspecifics and considered excessively warlike. The makers took some of their leaders to a maker ship and - fixed them. Made them light instead of dark. Kind instead of warlike. Patient, nurturing, peaceful, almost serene. Then they returned the healed hora quan to their home planet, where they were met with horror and confusion and then, eventually, hatred. Their own polity dissolved almost overnight. Some of its former members joined the polity they'd been trying to destroy, but not enough to make them a majority and not many who had been in decisionmaking roles.
The polity that the makers had protected by doing this declared war on them. The leader of the group of hora quan whom they had saved from annihilation proclaimed the healed hora quan an abomination, and the makers likewise, and insisted that the only fair and proper response was genocide. Initially they may have intended to aim at a smaller political unit, but later they learned that the makers were not at war among themselves, and so the hora quan aimed for the destruction of all makers, everywhere.
Well, not all hora quan. Several other polities lent either material support or manpower to the one that had declared the war, but no others declared war themselves. The war quickly moved into space and away from the hora quan home planet, and the makers left that planet alone during the war, other than surveillance and destroying any spacecraft they tried to launch. There were eventually attempts to retaliate for the spacecraft destruction, but not especially effective or disproportionate ones.
Attempts to ask any group of hora quan what they had done to deserve genocide generally got the answer that they had done something abominable in trying to heal the hora quan, who wanted to be left alone and allowed to continue being evil, and that it was just and right to exterminate them for trying to spread light and healing.
There's a description of various battles and the heavy losses the makers (and, for that matter, the hora quan) took during the war, and the only partly related shutdown of the rift network and the eventual success in sealing their enemies away inside it. By their own account the makers tried pretty hard to avoid doing anything they considered terribly evil in the war, while the hora quan didn't seem to have any qualms about things like attacking medics and damaging life-bearing planets and employing dark eco as a weapon. Descriptions of ship-to-ship combat and space logistics and so on take up about half the book.
Meanwhile, the dark makers (mostly not appearing in this book but evidently very important) killed the makers guarding the hora quan home world and then destroyed the entire planet.