Necromancy was kept secret by the imperial authorities; all knowledge was stored in the Black Archive beneath the Imperial Palace.
Emperor Radas III contracted the plague; by that time, a large part of the capital's inhabitants had already perished, and the coastal citadels were completely deserted (magic was respected in the empire, and all the inhabitants of the center were mages or people with increased magical power). The magical surveillance systems sent a warning to the capital about the approaching demon fleet. Radas decided to take extreme measures – to become a lich and raise an army of the dead, which he succeeded in doing. Now he was known as Radas the Last.
When the demons were destroyed, Radas realized that he had lost his empire – his subjects were bone puppets, and living people would not recognize him. Then, the army of the dead stood guard over the imperial lands, and Radas himself shut himself up in his palace.
Fifty years later, a group of Nord heroes gathered an army to destroy the blasphemous king-necromancer. Their army defeated the undead, and the heroes defeated the lich, but before he died, he cursed them.
The lands of the undead became the domain of the heroes; their Clans settled in the abandoned imperial cities. After a few years, all the heroes fell ill with an unknown disease and, after death, turned into higher vampires.
Now, 3 first-blood vampires have survived – these are the higher lords.
From them come the second-blood vampires – death knights and necromancers.
The third-blood vampires include the Nosferatu outcasts – powerful infantry.
Ideas
The undead can control living people – the vampire lords remained true to their oaths and continued to take care of their Clans.
Liches may differ from those in the West – the fact is that the lich elixir is made on the basis of Namtaru poison, while in the East, it is made according to Shantirian formulas (I remind you that the Shantirians walled up Namtaru's mother in a fortress and placed Nagas to clean out the evil forces with a magical staff)