[ WP ] In the future , science has found a way to cure death , allowing people to become immortal . The only way to get the cure is to have your reproductive organs removed , and never bear children .

( ( I'm going to ignore the science-fiction part, for this one. This post is more of a worldbuilding exercise than a narrative exercise. tl; dr at the bottom of the post. ) ) 
 
 `` So, let me get this straight. You want to offer me immortality.'' 
 
 `` Yes,'' sang the naked, radiant figure floating before him in the most cheerful of tones. `` All you have to do is sacrifice your potential essence to my divine patron, and you will form within your soul a self-sustaining microcosm of endless samsara that will-'' 
 
 `` I got that. I just want to know- why balls? Why ca n't I sacrifice my eyes or a gallon of blood or something? I heard blood's a great fuel for this kind of shite.'' 
 
 `` Well... Our Lady is a goddess of fertility, so naturally the reproductive organ is the proper sacrifice. Remember what it felt like, to be young and full of energy? Would n't you want to be like that again, forever?'' She smiled sweetly. 
 
 The man simply stared at the luminous elfin form before him, his expression betraying more than a little confusion. 
 
 `` And if you join us, you'll get just that- plus even a little more, if you're lucky! Any excess blessing you get will spontaneously collapse into a low-order configuration of- sorry, just think of it as magic. You get immortality, and you get magic powers as well if you're lucky.'' 
 
 `` Which is... why you're glowing and floating.'' 
 
 `` Correct! Will you now accept our blessing?'' 
 
 The man stood silently for a moment, before he spoke again. 
 
 `` I know that you probably think I'm some sorta idiot or something, but I ai n't. I did n't go to the mage academy, but I know a bit about magic, and about Gods.'' 
 
 `` Oh?'' Amusement. 
 
 `` Firstly, why would a Goddess of fertility want her followers to remove the implement with which the act of fertility is performed?'' 
 
 She tittered. `` Well, because-'' 
 
 `` Secondly, you ca n't make anybody immortal with a ritual. Nobody can- not even Gods. They've tried, but they ca n't, because of the constant state of leakage every organism's soul is under. And it's funny you should mention the sacrifice of reproductive organs, because...'' 
 
 The divine emissary was still smiling, but it was a slightly more strained smile. 
 
 ``... Because the reproductive organs, as a side-effect of their constant production of gametes, drains a tiny hair of life-energy from one's eternal soul over time to feed it to the tapestry of fate, which is used to maintain the cycle of reincarnations. At the moment that one hits puberty- when one's reproductive system really starts working- that tiny hair of energy you've given up in tribute makes you an imperfect mortal being, rather than the perfect being you were born as.'' 
 
 `` And stealing energy from the tapestry of fate... The tapestry of fate handles reincarnations. It takes care of us after we die and our souls pass on to the afterlife. Taking energy away from that means that we're taking away the energy that lets us reincarnate. Like pulling strands out of a massive woven tarp stretched above a bottomless pit, which we all happen to be sitting on.'' 
 
 `` I'm not sitting on anything,'' the elfin figure giggled sweetly. `` I'm floating.'' 
 
 `` Well... Yes. I suppose you are floating, both literally and metaphorically. Where other people need the tarp to keep them from falling, you... you've made a rope out of the strings you stole and tied yourself to the ceiling? Is that a proper metaphor? Whatever. Point is, your so-called blessing is just a defection from cosmic order. Take away a little bit of everybody else's metaphysical stability to ensure your own existence.'' 
 
 The floating form before him did n't respond, so he continued. 
 
 `` And that's not even all of it. The tapestry is composed of souls, first and foremost! That's what they teach first in magic school. Damaging the cycle of reincarnation means, obviously, that reincarnation will start failing. More stillbirths, more birth deformities- people who were fated to have normal children will now be bereft of their opportunity. This practice will directly lead to thousands of heartbroken families across the world!'' 
 
 `` Bereft? Such elegant words, from a mortal such as you.'' The elf was no longer amused, it seemed. 
 
 `` And that's... why... you need my nuts,'' the man realized. `` The ritual must begin by recalling all of my soul-energy spent on infusing the tapestry with potential descendants. It uses the reproductive organ as a sympathetic nexus to call back all the children and grandchildren and grand-grandchildren I could have had, wiping them from all possible futures in the process, and returns me to my prime in doing so. Then it steals some potential children from other, random families as well, and adds them to my soul in order to seal the leak. Your levitation and hovering are the emanations of souls being used as fuel,'' he finished in horror. 
 
 The figure opposite to the man stiffened, for a long while, then finally relaxed- and started laughing. A true, hearty, malicious cackle, not some false pretense of fae amusement. 
 
 `` You're right, of course. Good work! Obviously we made a mistake, in thinking you to be a mark easily convinced to bow and worship some new Goddess. Of course, that does n't change the fact that I'm the one here with the magic powers and immaculately perfect physical form, and you're just a mortal who's had too much alcohol for his liver's own good.'' 
 
 She grinned. 
 
 `` So now the offer has changed. I'm going to give you immortality, whether you like it or not.'' And with that, a glowing blade of radioactive green energy- a Soulblade- sprouted from her open palm. `` Join us, or die. Our faction is in need of those with insight such as yours, and you know too much to be allowed to leave.'' 
 
 The man smiled a wan smile. `` Truth be told, between the choices of living forever and dying here and losing my soul to a Tapestry that I now know has holes in it, I'd love to pick the first one. But I ca n't.'' 
 
 `` And why is that?'' frowned the immortal. `` I do n't see why-'' 
 
 And that was when a different Soulblade made of pink light burst through her chest. 
 
 `` Because while I did n't go to the mage academy, my son did. And I love him too much to cut him out of the Tapestry of Fate.'' 
 
 The false emissary gurgled, but no words were coming. The Soulblade had neatly severed her soul, and the so-called immortal slipped off of it and died on the floor. 
 
 Behind her, a hooded figure quietly put away his wand in its holster, before pulling down his hood to reveal a head of brown hair. `` Hey, dad. Nice job delaying her until I got back.'' 
 
 On the ground, the dead immortal's body had melted into a cloud of green, greasy fog. The mage reached down, and put both hands into the fog, which proceeded to suck up the fog like a miniature twister, leaving no trace that anybody had ever been there. 
 
 `` Did you get all the information you needed from her lingering soul?'' The man asked his son after he finished watching the vacuuming process. 
 
 `` Yeah. Four more Immortals, in the city. We can handle them. Even if they ca n't be hurt with mundane weapons, four Immortals ca n't possibly stand up against the force of the entire Academy.'' 
 
 `` The headmaster knows, then?'' The man supposed it was n't all that surprising, really. Headmasters did tend to be the kind of figure that knew all sorts of things they ought n't know by all rights. 
 
 `` Oh, he definitely knows. He's one of them. This is going to be a tough one. I'm probably going to need your help again to take him out. You up for it, old man?'' 
 
 `` Sure. Ca n't let you do these things all by yourself, kid.'' 
 
 ( ( tl; dr: The immortality process works by fate magic. By using your reproductive organ as a sympathetic reagent, you can steal the fate/life/energy of all possible descendants of yours, erasing them from all possible futures in exchange for ensuring your eternal life in the present. Doing so is also really bad for the cosmology, because it damages the reincarnation system of the world. ) ) 
 
 ( ( This idea actually started as `` what if liches had to use their nutsack as their phylactery because it's some sort of locus of life magic which nothing else can replace'', but it went a bit off from there. ) )
