Aphelion

They were aided by (1) their talent for mimicry (perfectly duplicating voices and speech patterns and accents) and (2) the fact that servants were practically invisible to the upper classes. Magic doesn't hurt, of course...and neither does genius-level intelligence (learning enough from their first victims to gain entre to society; from there, observing and absorbing knowledge of What Is Expected, while shopping for their NEXT victims).

Well, the place this one fell apart for me was when the head-hunters took to conversing in English without knowing the meaning of the sounds they were making . . .

Rather clever, otherwise.