This is something that came to me in a dream recently.
The ideas in my dreams aren't usually this coherent- this was an exception.
Elves claim to be blessed by the gods with biological immortality.
This is a lie.
Centuries ago, elven wizards perfected the technique of transplanting a soul into a new body.
Through this method, a person could evade illness and old age indefinitely, as long as fresh vessels
were available to them.
Of course, this results in the destruction of the original mind that occupied said body, and the displacement
of the soul to... wherever they go after death. There's no way to restore the first consciousness.
The process works best when the body donor is both willing and physically similar to the person
overtaking them. Because the donor needs to be awake and aware at the start of the ritual, resisting can
pose an issue, and a body that's too different to what the new mind is accustomed to can create
integration problems (roughly analogous to phantom limb syndrome or gender dysphoria).
For these reasons, the process of preparing a donor body starts eighteen to twenty years before the
transplant takes place. A clone is gestated inside a chlorodelphys tree, then raised to peak physical
health and conditioned to believe that having their mind overwritten will be a heroic and
necessary sacrifice. For those who can't afford this process, or whose need is unexpected, there are
always young people on the streets of the larger cities who no one will miss.