If Immortal were to die, her soul would last for as long as she lived after death. If someone takes her soul and absorbs it, they will stop aging, they will also take on some of her traits like her personality. It takes a while but their physical appearance will change to look like hers, they will also gain her memories and such. Soon enough, they'll become a reincarnated version of her, but if she was their kind. E.g, Sans takes her soul, in a few centuries he would look like a skeleton version of her. Their personal memories will still stay, they will keep their traits and such, but Immortal's soul would changer their appearance and personality to be like her.

If her soul is left alone, it will compose a new body for her. One just like her younger self, it's likely to be made from dust. She will have amnesia for a time, but within a century she'll regain them all. She will also regain the powers she lost when she was young, and it is unlikely she would only turn back to her modern form if she doesn't have traumatic experiences again. She will also be gender neutral for a term, until she decides on one manly based by the influence around her. She will also go on a hunt for her surviving friends.