There are three types of immortals in Allwind.
1) The reincarnant. This is the elvish way. Their souls get extracted upon death of the body, and placed into a fetus, that hasn't developed it's own soul yet, in early stages of pregnancy. Generally this requires a perpetual soul, that being, one that doesn't dissipate after the death of the body, which is achieved theough witchcraft. Essentially the soul, which is usually this ephemeral slimy thing in a skull(brains don't exist, souls are a like a glowy lil thing, that can be of various bright colors, they're in the skull), is trreated in a way, that makes it momentarily soludify into a crystal about fistful of suze upon any disturbance. This means either skull is unsealed, or death. The crystal is much more resilient and can keep it's form and personhood indefinitely. Of course, actually reincarnating someone requires the body to be a close relative of the previous body, so that it doesn't reject the soul. Usually reincarnants are their own grandchildren, but great-grand, nieces and nephews also work, mostly.
2) Ghouls. I am not very creative with names, bear with me. Every person has something called the seed, located near the bellybutton, burried between the guts. It stores equivalent of genes, it's fi sci, cells aren't a thing. It's very resillient and small. But, if it's somehow removed from the body, or damaged — the latter usually caused by extreme heat, the body no longer knows how to rebuild itself, and slowly rots away. Now, this is why organ transplantation generally doesn't work — they rot away. However, if you somhow also manage to obtain a seed of the person you take the organ from, you can simply put it there too and everything will be magically fixed. As a bonus, the fewer body parts a seed guides the repairs of, the longer it lasts. Of course, if you replace to many body parts to quickly, you risk the soul giving up and rejecting the entiee body, causing permanent death in non-reincarnants. Ghouls perpetually replace their body parts and organs, and take their victim's seeds. They can have dozens at a time. The victims die. Ghouls are often covered in scars, where parts of different people were stitched together.
3) Chimerizers. Chimeras are beings with more than one seed, kind of like irl. They usually live longer than others. Chimerizers are able to incorporate new seeds into them and grow completely new organs, which sets them apart from ghouls, who have essentialky normal biology, and so can only heal or grow organs they already have. Chimerizers usually get a modified uterus/ovaries, wether it's their own, or somebody elses, the modifications prevent pregnancy from occuring, instead the seeds created are absorbed into their body, allowing them to replace seeds as they wear out by simply having the right type of sex. This isn't always the case, sometimes they get underdeveloped fetuses implanted, that then get reabsorbed, leabing their seed behind, but that's considered a gruesome, immoral practice. Chimerizers can chimerize themselves continuously, and stay young indefinitely. They also get their scars completely healed over long enough times, something which ghouls usually don't do, because they usually can't. Reincarnants just get a brand new body...
Oh yeah, i guess this does mean. Chimeras who aren't chimerizers, generally have multiple biological fathers.