The word "fulfilment" is an abstract noun that can be expressed as a verb. Alternate translation: "to perform the duties" (See: [[rc://en/ta/man/jit/figs-abstractnouns]])
This can be stated in active form. Alternate translation: "You must kill any foreigner who approaches the sanctuary" or "Any foreigner who approaches the sanctuary must die" (See: [[rc://en/ta/man/jit/figs-activepassive]])