Generally speaking, I do not like writing the "ancient history" of my settings. I find that it often offers little return on time spent when it comes to actual play. However, in this case I've had enough people using Krevborna ask me for more information about the Vlaak that I find myself willing to bend my own rules.
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The people of the Vlaak Empire originated as an ancient folk capable of traveling the planes of existence. Rather than submit to life as servitors and prey for the cosmic horrors from the Outer Dark that conquered their world, the progenitors of the Vlaak fled into the many worlds of the Mortal Spheres in their planar-faring craft. These alien beings now exist in a state of diaspora. When a fleet of their ships find a world suitable for colonization, they invariably attempt to defeat the native population and found a dynastic empire that echoes the former glory of their homeland.
When these planar travelers descended upon Krevborna they displaced both the pomenysh and humanity with their advanced technology and strange sorceries. In Krevborna, the Vlaak Empire arose to dominate the land for a thousand years until it was weakened by decadence and hedonism. The Vlaak priestly class fell into the worship of Dracula, who influenced the Vlaak to adopt the fouls arts of necromancy and blood magic. In particular, Lithka, the last empress of the Vlaak Empire, embraced transformative necromancy in her foolish pursuit of eternal life.
It is widely believed that the Vlaak died out and that their empire fell before the rise of mankind, but this is a historical inaccuracy. The human beings living with boundaries of what would become the Vlaak Empire were enslaved as servants of the Vlaak. Hidden within the Grail Tombs is evidence that the ultimate downfall of the Vlaak Empire came about due to a revolt by humans the Vlaak kept in abject bondage.
Furthermore, the blood-drinking rites practiced by the Church of Holy Blood are a debased tradition passed down by the humans who served the Vlaak Empire. The faith’s sacraments are based on distorted memories of Vlaakish blood magic. A few senior Church officials have discovered the awful facts of their religion’s origins, but they dare not express the truth for fear of toppling the Church from its corrupt foundations.