The Sorinar were the accursed Inar, which came to be known as vampires among the common folk.
The Curse
As the Luinar people began to gravitate more toward a nocturnal existence, they discovered that they could access the Moon Lay via Ghost Rings. These were known as Moontreaders. Some of these he even more ambitious, and attempted to access the Celestial Lay in the hopes that they could become godlike. Instead, they mistakenly accessed the Shadow Lay, where they garnered the immediate interest of the Fouled Seven, who were trapped there. Recognizing that they could steal some of the inar vorsys, the Seven conspired to manipulate the luinar visitors into bringing more of their kind, among other creatures. The Seven believed that with enough vorsys they could have the power to escape the Shadow Lay. But they could not take all of the luinar vorsys, as they needed to be able to return to the material plane.
As a result of this loss of vorsys, the luinar changed. They were confined to darkness, and the sun would harm them. They maintained their extended life spans as usual, though they were somehow...lessened, more of the Shadow Lay than the material. They walked the world almost like the undead.
To compromise and keep the luinar satisfied and amenable, the Seven bestowed upon them otherworldly powers, such as unnatural strength and speed, as well as the gift of persuasive charm. They felt less pain, and could see in total darkness. Their senses were heightened. The Seven wanted them to be exceedingly compelling so as to attract more who could serve as a source of vorsys, so they also gave them exceptional beauty. These luinar also did not need to eat as they had previously, but began to thirst for blood, which they required to maintain existence while in the Material plane.
Rivalry
The sorinar immediately assumed that their druinar cousins of the Emra Gohg would be eager candidates, but they were mistaken. While the Emra Gohg were jealous of their relationship and access to the Fouled Seven, who they worshiped, they did not like that the change would make them less susceptible to pain, which they regarded as sacred.