The Undying Kings of the Underworld

In the aftermath of the Titan War, with the withdrawal of the gods left from the world, humans were able to seize divine knowledge and tools as spoils of war. This theft would reverberate even far, even to the gloomy halls of the Underworld. 

There were the recently arrived souls of great mages and heroes, fallen in the war either fighting for the titans or the gods. These personalities burning with power and rage at the perceived injustice of the universe, resisted the pull of dissolution. Their souls would not enter the cycle of reincarnation. Without the Unseen Lord, God of the Underworld, to quell their rebellion, they ransacked the cities and forests of the lands of the dead to find a way out. Their frenzy cracked the Underworld itself, and they found themselves on the event horizon of the Negative Energy Plane. 

They stared into the abyss and the abyss stared back. And then, perhaps, something within it spoke. The great mages and heroes discovered a way for their intellects to live beyond death, and even to be returned to their physical bodies. To power these miracles, they would need to build receptacles to house the negative energy. The only accommodation necessary to utilize this new and seemingly endless replacement for their life force was a small one. They would just have to give up their souls.

These nine (so it is believed) Undying Kings, or Lich Lords as they are sometimes called, became the new rulers of the Underworld. When that was accomplished, they sought to extend their reach to the world of the living. They sent tendrils of Underworld rising up, and the swirling mists formed cysts of the land of the dead, the Shadowfells. From these bridgeheads, the lich lords raised armies of undead, led by mighty warriors, called Death Knights.

Their conquest of the world of the living isn't driven merely by a desire for territory. Their goad is the Negative Energy Plane, and its hunger for unmaking and entropy. Their own fates are tied to its eternal appetite, so it's needs are their necessity. The self-centered nature of the Lich Lords has hampered their cooperation, and that combined with the weakening of the Shadowfells in daylight hours has slowed their advancement, but the Undying Kings are not hurried as time is something they have in abundance.

This is part of this setting.