The Blood Triangle is a roughly triangular area of Tha'lith with its points being the eastern mountains of Kilth, the swamp Gurutharni and the entirety of the Zan Urk mountains. It got its name during the reign of Varelay with the demons coining the term for the vampires that hunted in that area.
This area has long had a vampire problem, dating to the fall of Kezgihr and him becoming the first vampire. In the Lith-Crillion Era, the vampires that preyed on the demons of Varelay were the descendants of the Forsaken Six and their master vampire Kezgihr. Varelay built Drachlaz to handle the area's vampire infestation, and to perform experiments on those captured. These experiments on vampires were for the purpose of war, like blood starving and then turning the feral vampires loose against their enemies, or mixing demons with vampires resulting in the first bloodbag demons. At the time, the leadership of the vampires was based out of Zan Urk. After the fall of Varelay, the vampire's center of operations moved to Drachlaz.