Highlander (1992)

Taken from the film, Highlander, Duncan MacLeod, clansman of Connor in the film also finds he is being stalked by not only other immortals trying to kill him before the time of the gathering, but also a secret society of mortals who call themselves 'The Watchers' and also seem intent on killing him but "The Watchers" observe and record and never interfere. Duncan and the other immortals can only be killed by decapitation and often live for centuries.