There is an emperor of the world. Like this modern, real world, except this guy magically underpins the very fabric of the universe. He took over, about 6000 years ago, overthrowing some other ruler about who little is known. He has powerful allies, beasts, planets, sometimes humans but not usually.
His power is in magic, mostly in the form of knowledge. The language of beasts. Incredible healing arts. The ability to walk the lands of the dead and resurrect others or possibly yourself. Stored in a vast library.
For complicated reasons, he has an identity as a regular human in a suburb somewhere. Hosts barbecues, etc. His neighbours are, unbeknownst, some protective camouflage, if any enemy finds him.
He ends up adopting twelve children, and raising them, each being taught one of twelve major fields of study. There is a lot of being sent off to live with powerful sea creatures, or live in the woods with deer, or study endlessly under his guidance until you know whole books.
The novel skips through the early years, and then the bulk of it is what happens when the emperor goes missing and his children try to deal with it: there is a lot of one of the quieter ones trying to juggle multiple balls of intrigue amongst all the others.
I loved, loved, loved, loved the worldbuilding. It was a marvellous marrying of a traditional academic-y magic education, like things from Dark Is Rising, or Earthsea, or Once and Future King, with an almost urban fantasy setting of these ridiculous people trying to juggle alliances and also get familiar with mainstream american society.
The first caveat (and content warning) is, their upbringing is very abusive, both emotionally and physically, both from their father and some of the other siblings, including some occasions of torture and sexual violence. This is very well written, and it doesn't linger on it gratuitously, but it was still pretty heavy.
The second caveat is, I loved the worldbuilding and inter-sibling politics more than the overarching change-the-whole-world stuff. Some parts of that worked very well (when the emperor's rule overlaps with mundane politics, for instance). Others were ok, but just felt like they were too much for the book to carry.
So, if you like the magical library and people training in specialities with animals and learned other powers, this is a book you might REALLY REALLY REALLY like, or you might not.