Notes to catch you up: This is a world where the ultimate creative force is the Maelstrom. In the beginning, it birthed Nahadoth, the dark god/dess of chaos and change. After a few aeons it birthed Itempas, god of light and order. They fought and loved and fought and loved and after a few aeons more, the Maelstrom birthed Enefa, goddess of life and balance and all that. In pairs and all together they birthed many godlings, male, female, and other; they created Existence and Enefa peopled it with mortal things like stars and planets and living things, including humans. The eldest of the godlings was Sieh, a god of youth and mischief. He played a trick that set the Three warring against each other, resulting in the death of Enefa and many godlings, and the enslavement of Nahadoth, Sieh, and many other godlings to a family named Arameri. With the (unwilling) help of the gods, they built an empire called the Bright that brought peace on the world for two thousand years, which they ruled from a huge palace, on an immense pillar, called Sky..
Then a half-blood Arameri named Yeine ascended as a new Gray Lady, freed the Arameri's god-slaves, and punished Itempas by condemning him to walk the earth as a powerless mortal (except that he could not die, or at least stay dead). She interlaced Sky with an equally immense World-Tree.
Okay, you're caught up enough.
Sieh narrates this third book of the trilogy. He comes to play with twin mortal children on the Nowhere Stair of Sky; he would almost as soon kill them, because they are Arameri, but visits them once per year. He plays a deadly game with them, at which they surprise him by winning. For their prize, they request that the three of them make a blood vow of friendship.
But just as the bond is sealed, there is an immense release of magic. The two children are nearly killled. As for Sieh, he awakens several years later in the bosom of Nahadoth, who loves him as a mother. Nahadoth has protected him while he healed; and Nahadoth and Yeine are searching for a cure for Sieh's condition: he is becoming mortal, aging and losing his godly powers. When he returns to Sky he finds that the boy twin (Deka) has been sent away to become a scrivener, the mortal way of magic, while Shahar, the girl, is being groomed for headship of the Arameri clan.
Sieh discovers that someone is murdering members of the clan. There is no common cause of death, but the dead are always found wearing masks, their faces burned badly. Then, at the behest of her mother, clan leader Remath, Shahar beds Sieh, hoping to breed a half-godling child (a demon in the language of this world). Feeling betrayed, he decides to murder Remath; Shahar stops him, but he leaves Sky, to live in Shadow, the city at the foot of the World-Tree.
Here he meets Ahad, a young god formed when the enslavement was ended, and Glee, daughter of Itempas (this is explained in the second book). They send him a'spying, and he finds a conspiracy determined to overthrow the Arameri by use of magical masks; he also discovers a godling he does not recognize, but who says he intends to kill Sieh as a matter of vengeance.
So that's about the first quarter of the book (and I've left out a lot, obviously) and I'll stop summarizing here. I'll add that Sieh, Shahar, and Deka are three of the most engaging characters I've met recently, and that their struggles with and against each other _matter_ in a way I rarely find in fantasy. I find the final chapter a bit of a stylistic cheat, as the narration suddenly shifts to another character. This is somewhat redeemed by the Coda, but it was severe enough to briefly break my "fictive dream". In the context of the immense Inheritance trilogy, this is something of a trifle, though I find it one worth mentioning.
The Broken Earth trilogy (which is really a single long novel) convinced me of Jemisin's importance as a writer; this book confirms it. Long may she write!