After the Primordial War, everything was ruled by the Solars. Except that the Solars went insane, so the Sidereals did a line of coke off the Loom of Fate and decided they should kill all the Solars and install the more stable (and easily-manipulated) Dragon-Blooded as rulers of Creation. This worked, more or less. The Terrestrials weren't as good at it as the Solars were, but by and large they also didn't have the Solars' tendency to be mind-staggeringly bad at it when the mood struck.
This was all well and good until the Deathlords decided to unleash a massive plague and wipe out 90% of life on Creation, including most of the ruling Dragon-Blooded. And then they told the Fair Folk to go kill 90% of the remaining 10%, which pissed off the surviving Lunars, who held off the fae long enough for the Dragon-Blooded to find some WMDs the Solars had left lying around. One of those Dragon-Blooded found the WMDs, bound them to her soul, blew the shit out of the fey, and crowned herself empress of all Creation on pain of Solar-built super nukes.
And thus, the Realm was founded, plus or minus a few places that either weren't important enough to conquer, or maybe had their own Solar superweapons.
She ruled for 750 years, and things were pretty good, even for some of the mortals. And then one day, she decided to just fuck off. Without any clear succession, and a government more or less built to catastrophically fail in her absence, the Realm predictably fell into a shambles. Her children and ministers were too busy plotting against each other or just fucking around to actually rule the Realm, and collectively selected the single least capable person in their number to keep the throne warm. If this reminds you of anything, ask your nearest Immaculate monk to ease your doubts.
And then the Solars came back, and with them, the PCs. Gods help us all.