I am Josephine Mengalus, an alchemist on the path to immortality by becoming a lich. Alchemy is the study of combining magic with science, to create a higher form of study that surpasses both. Souls are power sources that possess large amounts of energy, and can be used for spells or rituals, or to power certain machinery based on magic principles. The souls id made up of two parts. The first contains the memories and personalities of the individual, eveeyrhing that makes them "them". The second contains the life force of the person, that is immortal and remains after death. I plan to harvest a large number of souls for my soul forge, a specialized container built to hold spirits. Through the use of a dark ritual, I plan to use this soul forge to elevate my to lichdom and make me a god among men. Of course, the devil is in the details, and I must arrange to collect a large number of souls in a short amount of time as well as remain inconspicuous from prying eyes.

My first thought was to raise the souls of the recently deceased. However, souls maintain strong connections to their bodies after death, making it almost impossible to remove. That connection fades the longer a person is dead, bringing me to my second idea of raising the souls of the long departed who died centuries ago. These souls are "blank slates" who have lost all memories from life and have no remaining connection to their physical forms. However, the longer a person is dead, the harder they are to raise. It would take a massive amount of time and power to harvest. Which brings me to my last idea: harvesting the souls of unborn children.

It takes 9 months for a child's physical body to be created, during which time the soul develops last in the third trimester. These are the Goldilocks souls: blank slates that lack any attachments or memories, and new enough to be easily harvested. I cast a powerful spell that magically harvested ( or "murdered", for the politically correct crowd) the souls of millions of children across the country, drawing their souls into my forge.here are a few alchemists of my craft who would protest this on the grounds that casting such a spell in the name of science violates certain "moral" principles. As idiotic as these excuses may seem to you, they have highlighted a problem.

The spirits of these children are known as half-souls. These undeveloped souls, while malleable and easily taken, do not possess only half the volume of energy as a full soul. Logically, I assume that stitching and combining a number of half-souls together would increase their volume and provide for a higher energy output. However, this is not the case, as the quantity of energy that the souls output remain the same. How can this be the case?

Because souls are like stars

If you combine enough stars together, you would get a black hole, one that is significantly smaller in volume but more massive than any of its constituent components. Also, the black hole emits far less energy than a star.

This is just an effect of a physical law of nature, gravity. If magic or alchemy or whatever laws govern souls in your world have an attractive inverse square law with distance, then you would get the same effect. Tinker with the value of the universal constant of soul attraction and combining as few as two souls could give you a black soul-holes. Or something.

Whereas 10 souls might output 10$x$ units of soul-energy, one black soul outputs nothing. Unless black souls also have soul-Hawking radiation or soul-accretion disks. Or something.

Decay magic. (As in nuclear decay, not rotting away.)

Since you can't touch the soul directly, you can only measure it by its energy output: how much magic you can do with it at any one time.

These "half" souls are already fully formed. Combining them does not increase the energy density because the space in the aether where they exist is already filled up. I.e., if you pour two gallons of water into a one gallon bucket, you still only have one gallon of water in that bucket. They don't look like they're fully formed to you only because you can't see them directly.

Now, as a soul is used, which normally happens by animating a body, part of that pure soul energy breaks down into a less pure, more tangible form. This exercised bit of soul energy continues to release more and more magic until it gets all used up over a couple of decades. Once a bit of soul starts emitting magic, it doesn't stop; it takes a long time to release that energy.

This is why a freshly harvested soul from an adult is so easy to manipulate; it's full of energy that will take a long time to dissipate back down to background levels... but because the person using that soul has exercised bits of that soul in certain ways with certain habits and routines, the most energetic parts of the soul match those habits and routines as well... A caretaker's soul could be used to cure wounds, a warrior's soul can be used to cast fireballs and magic missiles.

An infant soul, though, has very little of its soul exercised; no habits or routines yet, and it's "cold" -- not emitting any residual magic yet. Necromancers of the past have been unable to jump-start the reactions, and haven't had the patience to let the soul "warm up" before discarding them as failed experiments.

Careful about getting too many infant souls together in one place, though. Once they start emitting magic, they won't stop for many decades to come. You need to keep using the magic so that it doesn't accumulate in dangerous ways...

This is similar to uranium in a power plant, which is fairly non-radioactive outside of a power plant. The uranium produces prompt radiation with its initial fusion due to being close to other uranium (and several other factors that I'll handwave for brevity)... then going on to produce additional heat through delayed radiation, as the daughter products are still highly radioactive themselves, but take hours, months, and years to decay into stable lead, all the while releasing energy long after that first prompt reaction.

How many fetuses does it take to change a light bulb?

None, fetuses lack the basic motor skills to change a light bulb and therefore are useless to all such goals. Adding more fetuses isn't going to make the job more probable of happening, it's just going to cover the floor with more mess that your janitor is going to have to clean up.

No, you need souls that have basic competence, but not attachment.

Souls don't increase in volume because volume is a property of matter, something which souls, which are demonstrably not made of matter, do not possess. Souls have altogether different properties, and the additive property isn't one of them.

Simply put, soul arithmetic doesn't follow the laws of normal arithmetic.

They are not actually souls. Soul is associated with the "breath of life" and children receive it with their first breath in after birth.

What develops with the last trimester and what your alchemist is collecting is the receptacle for the soul that contains the soul after the child receives it at birth.

Combining several receptacles into one leaves you with one receptacle and hence one soul.

What you want to do is find a way to sustain the receptacles separately and let each separately gain a soul which you then suck the energy of. You then combine the energy. The order in which you do things matters here.

It might be simpler to find a way to suck the energy of a soul without killing or breaking the vessel and then use a ritual to open the vessel so it can receive a new soul which you can then also harvest. You could in theory repeat this several times per vessel as long as you leave time for the vessel to heal between operations. Potentially without killing anyone too.

You can also just harvest a portion of the energy. Harvesting one third of soul energy of triple the people should give the same result without permanent negative effects.

Fresh souls lack the resilience of experienced ones.

An old soul, one that belonged to a person who had lived a long and varied life, has become strong and resilient to the ravaging effects of being detached from a vessel. Fresh souls however, are barely formed and gossamer.

When you harvested so many fresh souls, you expected a certain volume. However, by the time they'd been drawn in and housed in your forge, they'd lost much of their material to the inevitable degradation inherent in the exposure of a soul to the space between.

Note: For further reading on the principles of soul erosion, please refer to chapter sixteen of Thurixarion's Treatise on the Harvesting of Incorporeal Anima.