Blood control

Via blood control, an entity can have power over the donor of the blood and control their mind and actions. Blood control does have its limitations. The Sycorax could control every Human on Earth, in theory, however, as they only had a sample of A+ blood type, they could control only one third of the Human population.

The Doctor said he had not seen blood control used in ages. (DW: The Christmas Invasion)


 * The Sycorax probably regarded blood control, like other technologies, as a form of witchcraft or sorcery.