The phrase "mysterious employer" refers to an individual who is hiring someone for a job, but their identity or background remains unknown or unclear. Full definition
His mysterious employers, thirty years in the future, send back his targets and he takes them out and collects his silver. (hoopla.nu)
Terminal tells the story of two hit - men (Fletcher and Irons) as they embark on a borderline suicide mission for a mysterious employer and a high paycheck. (max-irons.com)