Sombra's magic, the magic he used to torment and torture the living.
It's genius. Genius in a way that invokes terror second only to the spell itself. It narrowly averts all of the pitfalls of magical transfer between individuals, one after another. If it weren't so truly vile, I would be suspicious if he had even invented it himself. But what I've seen reeks of nothing more than his touch. Domination, sadism, and above all else, the ceaseless hunger for control. All of these elements enter into his Fear Absorption Array, and unlike his other spells and creations, this one lacks his flaws. There are no holes born from arrogance, no loopholes born from his massive hubris. Only the cold calculation of someone who has decided henceforth that the living are to be a resource.
It begins with the first key of magical transfer between individuals. Namely, it cannot occur. Magic passing from one pony into another will carry their imprint, making it little better than an enchantment, typically with dire effects. One simply draining magic from a pony would find themselves rapidly suborned to their victim's will. Instead, his spell drains away at something I had only been tangentially aware of originally. It drains away at their very life force instead, using a spell tailor-made to drain the life out of the terrified. Slowly, and insidiously, it invokes a subtle shift. Where before, a pony aged as their spirit grew weary, this spell interjects in that process, siphoning off that energy for oneself.
The second law of magic which it shatters firmly is the nature of manipulated emotions. Using a spell to alter the emotions of others is a trial and a half. And in most cases, the emotional state is shallow, and the bulk of the spell is in altering their behavior in spite of the low emotional state. Sombra's array bypasses this in two key ways, both of which are derived from the mind control he already has access to. Two simple commands empowered with sufficient magic are enough to do the job.
The first step is to suppress a pony's ability to convert short term memory into long term memory. This is a prep command, which makes them infinitely susceptible to the second part, and ensures that they are reduced to a quivering wreck, suitable for draining. The spell's second command is for the pony to remember terrifying memories. In the Crystal Empire, such memories are unavoidable, and all at once, they can turn any pony into a mindless, thrashing thrall. From there, Sombra's array only needs to keep them from injuring themselves, sustain them, and deal with any illness or injuries that may occur.
One thing I can't help but wonder, is why he even bothers. The spell he used to drain Life Force is modular, it can drain a variable amount. Why bother keeping the ponies alive and only draining the Life Force in such a way that it doesn't impact their actual lifespan? Is it a matter of efficiency? I doubt Sombra even knows what that word means. Is it a matter of morality? Some line between torture and murder that he isn't willing to cross?
I doubt it. What is far more likely is that this strange arrangement of his is devised to satisfy his core desire for control. The same desire that prevents Domination magic from inflicting death on an individual whom the user sees a use for. The magic rebels at the idea of killing something or even maiming it, because that would reduce its ability to serve the user in the future.
Another alternative may simply be that Sombra lacks a vessel to contain the excess Life Force. But if he's infusing it into himself for magical power, then why bother creating a vessel to contain it? He could simply infuse massive amounts of it into himself, leaving no uncertainty in storing it. If a vessel were to be created to house Life Force, then it would only be a subject of covetousness. I doubt there are any among the living who don't see the use in even a few more years of life.
Indeed, I'm not exempt. Unlike Sombra, however, I'm not going to be using Life Force as a fuel supply. I'm going to be using it to continue my duties. There is a certain amount of fear that emerges when I consider what will happen to my ponies after I'm gone. Ergo, I have little choice but to find a solution. While I could try to find a way to become immortal, that seems like the sort of ridiculous venture that would only leave me an elderly failure. Instead, I will twist Sombra's magic to my own ends, as I've done countless times before. A relic that will allow me to drain the Life Force from another living thing, and store it for my own uses.
I'll do some surveys to check the waters on this. Because the spell impacts short term memory, and there are no true side effects for the draining of Life Energy, then I can make use of it in a mostly ethical manner. I'll be straightforward, clear, and honest with my intentions and what the process entails. If ponies appreciate me supervising them, then they can donate seconds, minutes, or even hours to my lifespan, and remember nothing of the terrifying and short period required to drain it.
Is it unethical? God, I hope not, I'll have to put out some propaganda to absolutely and unequivocally stress the fact that such a donation is not only optional, but a massive undertaking, and one in which the pony will suffer extreme discomfort for the short process in which it takes place.
Maybe I'm being greedy, seeking a longer life at the expense of others, but the opportunity is there. It would give the Shadow Realm a massive safety net, and allow me to develop infrastructure that could help it survive in the event of my final demise. The fear only emerges from one admittedly arrogant thought. Could anybody manage this place as well as I do? Could everyone who ever takes my place do so, without fail? How many idiots running the Shadow Realm in the far future could the Shadow Realm endure?
I suppose there is only one way to determine if what I am doing is good or evil. I'll ask the people.