King Taravangian ponders that Nergaoul is an ancient, evil spren and one with which he hasn't the resources to tackle. Adrotagia implies that Nergaoul is a Voidspren, or perhaps an Unmade connected somehow with the Thrill.
According to Taravangian, the Thrill only affects people who are in a certain proximity to Nergaoul.
|“||"The Thrill is at least as strong here as it is in Alethkar. Maybe stronger. I will speak to our scholars. Perhaps this will help pinpoint Nergaoul."||”|
–Adrotagia to Taravangian
Nergaoul is indeed an Unmade connected with the Thrill. Brandon has confirmed that the Unmade are Splinters of Odium. Odium splintered himself and created the Unmade to be his influence on Roshar. It is through the Unmade that Odium may be able to influence people in how they think or act.