They were not immortal. They could be killed, but when one died another awoken. How particular people were chosen to be a part of the Five, I don't know. Their true purpose was to prevent the revival of a creature much older than me -- once that happened they became irrelevant.
One of the ones we ran into would use werewolf venom to poison vampires -- deadly to regular vampires.
[But for her it had just made her sick for a while.]