The Fatality Index was used by a species from Carnathon that regulated how that species carried out executions.
Missy was sentenced to death by this species and, in accordance with the Fatality Index, the Twelfth Doctor was chosen to carry out the sentence.
The Index can be used to look up how many beings have been killed by a single individual by searching for their name under cause of death. To intimidate the species, the Doctor had them look him up on the Fatality Index. The number of deaths attributed to the Doctor caused them to flee in terror. (TV: Extremis)