Requires the Monks & Mystics DLC to interact with multiple prisoners simultaneously.
Navigate to the 'Prisoners' tab on the Intrigue Page. To the right of 'Actions' is a green arrow, which opens up a filter tab, allowing you to sort the prisoners you want to process by sex, age, status (foreigner etc).
Simply select at least 10 prisoners from this filter menu, and then select the 'Execute' button just above.
(No, they don't all need to be decapitated, any execution method will do!)