"We are but servants of our patron, the Plague Doctor. We are in your hospitals, in your homes, in your clinics... Everywhere a doctor may be...we are there as well."
Oathbreakers are the servants of the Plague Doctor. There are two varieties of the Oathbreaker: Plague Bearers and Mad Doctors. The Mad Doctor is the highest rank. They will use their patients in morbid experiments involving reanimation, cloning, or anything else their minds can come up with. They are simply first observing their subjects, they analyze their every action and then, They do their job.
It is unknown exactly what causes the Plague Doctor to choose a Doctor for this career. Be it a remarkable intelligence and skill, a morbid fascination with the body, or even sociopathic traits, almost any doctor is chosen to be a servant, though not all join willingly. It is also unknown how one "progresses" through the ranks of the Oathbreaker cult, be it through loyalty, experimentation, or some other factor.