A person is born with a certain amount of mana, which increases with age and peaks at a certain point. Individuals are born with all the mana they will ever have, but it can be refined with practice and training. This will determine how well it can be controlled and used to perform spells.
However, there is a way to cheat this process. Mana is transferable through a dark ritual that involves torturing and killing a host. Through this process, a renegade witch can absorb the mana of others, making themselves stronger. These witches are called crones. Mana that has been honed with training gives crones the most power, making competent witches the most sought after targets. At a certain point, a witch would achieve apotheosis and evolve into a higher life form.
However, absorbing mana from others leads to physical changes in a crone, eventually losing all resemblance to humanity. Therefore, crones must operate away from society, and use proxies to obtain victims for them. The major problem here is secrecy. Criminal enterprises are most successful in limited numbers. The smaller the circle, the less risk with exposure, the more likely anonymity can be maintained. Large enterprises such as human trafficking is a momentous undertaking. This is a huge operation that involves many people, any one of which can alert the authorities or make a mistake, bringing the whole thing down. In addition, keeping the disappearances of prominent witches a secret without raising suspicion in a community is difficult.
While crones are powerful, they don't want to deal with entire bands of trained mages after them while their goal is incomplete. How can a crone reduce the risk of being caught with all these variables in place?