Blood control

Via blood control, an entity could have power over the blood's donor and control their mind and actions. Blood control does have its limitations. Even from space, the Sycorax could control every Human on Earth, in theory. However, since the Sycorax only had a single sample of A+ blood type from the Guinevere One, they could control the one third of the Human population with A+ blood.

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.