One of the things that defines elves in Eberron is their relationship with the past, and specifically with their dead. Elves live for a very long time compared to other races, and this means that the weight of their lives is heavier. They have real trouble letting go of the past, and this is especially true when it comes to people. If you've had someone around for the last seven hundred years, it would be really, really hard to let them go and accept that they weren't around anymore.
Every elven culture is defined by how they choose to deal with this. Those Qabalrin elves that survived their cataclysm shared territory with the Undying Court, but they had a different approach to preserving the greatest of their people. They saw the flaw with the Undying Court, their reliance on the island of Aerenel and the manifest zone to Irian.
Instead, these elves preserved their greatest souls by drawing on the energies of Mabar, reanimating their greatest people as vampires, liches, and other undead that sustain themselves by consuming the life force of others. Chief among these necromancers were the line of Vol. They held these beliefs and followed these traditions for thousands of years before they began to manifest the Mark of Death.
The Aereni believe that Mabaran undead drain the life force of Eberron to sustain themselves, even those that do not actively predate on others. The tension between the line of Vol and the Undying Court had been building for millenia, and then the Mark of Death manifested and cemented the line of Vol's influence among the Mabaran faction of elves. The line of Vol's research into undeath and immortality led them down questionable paths, in particular an alliance with a group of dragons from Argonessen.
This alliance eventually culminated in a program to crossbreed dragons with elves from the line of Vol, to see if a dragonmark could manifest on a dragon. This experiment eventually culminated in the birth of Erandis Vol. She wasn't the first half-dragon to be born, but she was the first and only of them to manifest the Mark of Death in her adolescence. Erandis didn't just manifest a normal dragonmark, either, her mark was more powerful than even a Siberys mark. It was the purest, most powerful incarnation of a dragonmark ever seen on Eberron.
People were scared. The Undying Court had begrudgingly put up with the line of Vol's use of necromancy for thousands of years, they had even put up with the Mark of Death for centuries. The manifestation of the mark on Erandis was a step too far, and the majority of the dragons of Argonessen agreed. Never before had such a potent dragonmark manifested, and the Undying Court took the position that the entire line of Vol was to be destroyed. If even a single drop remained, it could lead to the end of all life on Eberron.
The elves of Aerenel and the dragons of Argonessen formed a tenuous alliance in pursuit of this genocide, and they succeeded. The line of Vol was eradicated utterly, and those other allied elves who were not directly related to the line of Vol were given a choice: exile from Aerenel, abandon their traditions, or join the line of Vol and be eradicated.
Those elves who chose exile left the shores of Aerenel and sailed to what is now known as the Lhazaar Principalities, where they spread into ancient Karrnath. Here, they shared their traditions and mixed with the local population. They showed the humans how to tame and placate the Mabar manifest zones that litter Karrnath, and this mixing of cultures eventually led to the formation of the Blood of Vol.
There are still elves today who practice the traditions of the line of Vol in the Lhazaar Principalities.