The Isochrones or Amaranthine are a nephilim legend. They are one of those urban myths much talked about within that society. No-one has ever seen one, no-one knows what one looks like in order recognise it were they ever to be so fortunate.
If the Fallen are rock-stars in nephilim culture, the Amaranthine are the gods.
All nephilim are effectively granted immortality by the Golden Helix, except for those afflicted by strigoi-strain of course. Functional immortality comes with the genes. Old age, sickness and death are no longer things to concern them, their body will repair those for them. Heavy drinking and smoking and most drugs have little effect on them and certainly won't ever inflict them with fatal diseases. Shoot one in the head or cause them some other major trauma and most of them will die and remain that way.
Many nephilim have a healing factor of some description; wounds heal faster, severed limbs may be regrown, some even have almost instantaneous healing but this is rare. However even with those, if you remove the head, exsanguinate them or cause severe and traumatic damage to their body, they'll die just like any other human.
Not an Isochrone though. You can kill one of them as easily as you can one of their kin, that is true. The main difference is that if you kill someone they tend not to come back later and return the favour. In person. An Amaranthine is said to be able to recover from almost anything given enough time. Blow its head off and, if you leave it alone long enough, it will grow another one. Blow its body to bits and any remaining tissue will reunite and grow into a brand new creature. The oddest part, though, is that an Isochrone will regenerate with a completely different face to the one it died with. It will come back as a completely different person but with all the memories of the now dead person. In fact, they are the dead person, only they are now different in appearance. They are no longer dead either, of course.
Legend, myth and even popular culture are full of examples.
A certain British writer who was responsible for a well-known and long-running sci-fi show has claimed, to some sources, to have based his medically-trained time-traveller on a real Amaranthine whose resurrection he witnessed during WW2. Mr Newman, who died in Canada in 1997, never confirmed this story. It is not the first time nephilim 'intelligence' found its way into the public domain through fiction. Some wonder whether these 'leaks' are all as accidental as is claimed.
We know that Conan told stories of his early adventures and mythos to a well known Texan author as well as, it is deeply suspected, to another well-known wordsmith from Rhode Island. Those were not part of any grand plan, we are sure, but due to Conan's love for attention and alcohol.
Just like so-called accidental political 'leaks' though, we are sure these 'unintentional influence' on pop-culture and human thinking are as deliberate as Lucifer's masterful piece of work surrounding the existence of UFOs. There are still plenty of people who have no idea that he manufactured it all and that all the 'information' and 'leaks' were authorised by and often created by him personally. There are a lot of people who still believe advanced civilisations from light years away travel vast distances so they can violate - aha - country-folks then deface cattle and corn fields. Lucifer is subtle beyond almost anybody's ability to understand, that is what makes him dangerous. Have no doubt that he has been influencing human history, thinking, society and culture for a very long time now.
The Amaranthine may well be an invention of his, just like UFOs were. No-one has ever seen either.