Most nephilim or their descendants are immortal, at least exceptionally long-lived. Not those with the striga-strain though. Many compare striga-strain shifters to human anaemics or diabetics, their tertiary helix is flawed and fails to correctly integrate with their human DNA strands.
Strigoi are shifters though, due to the integration problems, this ability is rarely voluntary. Hunger, stress or other adrenalin responses can trigger such changes. These involuntary shifts among striga have been credited with myths regarding such creatures as vampires, wights, lamia, werewolves, etc.
When in stress-induced or feeding shift mode, the striga will assume a form which has commonly been compared to "a dog with its skin taken off" or a "pickled monkey" among other amusing images. Amusing as these comparisons sound, a feeding striga is not to be laughed at. Their jaws will lengthen, flesh ripple and change texture, dagger-like teeth emerge, many also grow scythe-like claws. It varies between individuals.
Regardless of appearance, strigoi feed upon human beings. They do not drink blood or consume flesh, they absorb souls. Commonly called soul-eaters, strigoi will kill their victim and, at the point of death, rip the soul of the dying man or woman into themselves in order to renew their own and keep mortality at bay for a few more years. Most strigoi must 'feed' every twenty years or so or they will start to weaken and age.