Originally Posted by
Vyrerus
But there is one hard thing the Sundering did that Rejoinings could never do. That is, it damaged the soul which is the core of an Etheirysian's being. At first it seems not to matter, we could even be generous and suggest that it's the same sort of deal IRL where the spirits housed in Eastern shrines are split and taken to other shrines to impart a growing spirit across all the lands. However, FFXIV's account of Etheirysian souls is too detailed for that to really hold water. Especially when the combining of the sundered souls restores power and vitality, as the soul approaches wholeness.
This(Ye Olde Sundering) made Etheirysians susceptible to, "true death" via Dynamis, and has lead to the only true deaths for Etheirysians with no possibility of reincarnation for such individuals (At least as shown currently, though I suspect with the writers wanting to maintain Hydaelyn's "inexorable" integrity they will throw the baby out with the bathwater yet again at some point in the near future. This will create its own host of new problems, probably).