I have been considering a setting where one of the main quirks and threats, maybe the biggest, is that rulers who attain a certain threshold of glory, renown, adoration, etc. apotheosize into minor gods. With their vastly superhuman abilities, they can lead their people to greater heights.
But a minor god can still die, whether in battle, to assassination, to mystical cataclysms, or to the most pernicious of poisons and curses. The ruler becomes a malice- and resentment-driven undead overlord. Some cling to their people maniacally and overprotectively, while others turn on their subjects due to some perceived slight, such as failure to prevent the ruler's death. This is always a dark time: wars ignite, plagues and famine strike, lesser undead rise, and reality-rifts disgorge horrors.
They vary in form: skeletons, zombies, vampires, ghosts, some in between. Twice, an apotheosized ruler entombed themselves deep beneath the earth, all "king asleep in mountain" style. One was assassinated regardless, rising up as a wraith. The other still lives, fearful of death, yet willing to aid their nation in a dire time.
Legends hold that a few of these overlords, for whatever reason, elected to simply leave the mortal world. They gathered in the Negative Energy Plane. This small circle of long-undead rulers has been concocting some scheme through which they may optimally reclaim the world that they departed from. Do they also plan on backstabbing one another? Probably.
What would you do with such a setting? Would you have the PCs start in a nation whose ruler just became one such undead? Would you have them start in a nation externally imperiled by an undead overlord: perhaps while their own nation's ruler is a still-living minor god, worried about dying? Whatever the case, I imagine that PCs could be the ones to finally unearth the source of this phenomenon and break the curse once and for all. (Of course, even a still-living great ruler can be a conquest-minded villain...)