The Elder Scrolls series is rich with history of its fantasy world, far more than is actually present in the games, hinted at and detailed via in-game books, texts, and lore notes, not to mention exterior novels and the like. The Crusader Kings series, meanwhile, specializes in the grandiose, attempting as it does to model the incredibly complexity of medieval politics over a span of hundreds of years and a region stretching from West Africa and Iceland to Siberia, Tibet, and Bangladesh.
It might not surprise you to find out that a lot of people who like one also enjoy the other.
Thus, Elder Kings 2, a sprawling and deeply impressive mod for Crusader Kings 3 that recreates "the world of The Elder Scrolls during the turmoil of the 2nd Era" which doesn't mean much of anything to me but I am sure as heck excited to play it. There's a snake-eyed emperor man, some cool rebels, an angry orc. There's elaborate cat-person genetics. I am here for it.
Elder Kings 2 introduces an elaborate system of magic, new and strange governments for inhuman cultures, new succession systems for feudal lands, Elder scrolls' Birthsigns. You can be a vampire too, of course. That's not to mention the obvious stuff just to make the world of Tamriel exist in the game at all.
Stuff like a massive map about two-thirds the size of the base CK3 map. The ten base Elder Scrolls fantasy races, and more apparently, like "Tsaesci, Lilmothiit, Goblins, Kothringi." Plus lots of new cultures, and a system for those cultures to have homelands true to Elder Scrolls lore, as well as rules for the various divine pantheons and patron gods of the fantasy world and their holy sites.
If this sounds familiar, that's because a similar mod was made for Crusader Kings 2. That was an impressive piece of work, but compared to what the team has done for this game it was child's play. The Elder Kings mod doesn't require Crusader Kings 3's released DLC, Royal Court, but the mod team is clear that some experiences in the mod—like the emperorship of Cyrodiil—are much better if you have it enabled.
You can find Elder Kings 2 at https://elderkings.com/. It's available for download via Steam Workshop or Nexusmods. There's a reddit for it, too, if that's your thing. The team does warn that the first time you boot it up the mod will take forever to load as it calculates shaders and textures—just the first time, though.