Heresiarch

Heresiarchs are to cultists what the magisters are to mages. The peak of their profession, something that roughly follows the same aesthetic and origin, but is something fundamentally different and much greater.
They are born from the particularly powerful cultists that has managed to gather more personal attention of their chosen Power , leading to them receiving something that isn't yet another Gift - but instead a total rework, creating them into something that's easily summarized as 'much more personalized demihuman'. They're humans only as much as magisters are, and like them they are too inhuman to be fertile.
Becoming a heresiarch locks you to a maximum possible level of Attention, which in turn implies major warping of their perception and minds to better fit their patrons. This makes them a bit similar to magisters 'frozen mindset', except heresiarchs are always much, much crazier about it.
Once one becomes a heresiarch, there is no step back. You can also expect them to spawn new cults for their patron wherever they are, even if the cult that officially elevated them into the position of a heresiarch has been destroyed.