The oprichniki operate as a kind of secret police and are culled from the First Army ranks by the Darkling himself. They are feared among Grisha and ordinary citizens alike because, though they are still technically the King’s men, they answer to no one but the Darkling and take both their orders and their pay from him.
They are extremely loyal to each other and to the Darkling. But in Ravka, it’s always wise to watch your back.
(And thank you for the kind words on the book!)