Mark of Indenture

A Mark of Indenture was a memetic marker which confirmed a deal between the Celestis and their servants. Everyone marked by the Celestis became a servant of them upon death. (PROSE: The Book of the War)

The Eighth Doctor was able to avoid being marked by the Celestis by having a Shift trapped in his mind; when Trask attempted to mark the Doctor, he missed and marked the Shift instead. (PROSE: Alien Bodies)