Yeah, but "I take pieces of your Hun soul to fuel mine" is something that... Well, like a lot of Dark Souls setting elements, it would work very well in a ghost story, used by ghosts against ghosts; you draw part of another ghost's substance from them, they lose memories, pieces of themselves, of their sanity, and in return you get to stay "normal" even as you grow in power, to avoid for a while the transformation into one of those twisted, molliated, Passion-mad wraiths that usually comes with ghostly empowerment. But it's not a "general purpose" thing; it only really makes sense as something used by ghosts againt other ghosts. You can't really make an Artifact, for instance.
Your problem here is that you think the protagonists of Dark Souls are ghosts, abyssals or undead.
They're not.
They're raksha.
Let us look at the facts, gentleman! (taps whiteboard with pointer)
Fact the first; the raksha are not a natural part of Creation. They require a constant intake of Essence to maintain their human guises. They can nto respire the Essence of Creation so what do they do? They
feed on the souls of humans.
What happens to raksha who do not feed? They slowly go insane as their Virtues hit their limits, causing them to suffer effects such as the curse of forgetfulness that makes them lose their very identity. Eventually they Calcify, and become reduced to basic elements. Some become corpses, others become humans, but humans that are mad and insane by the views of most humans.
Well, what happens to their victims? They too go mad! Their memories, emotions, eveyrthing that makes them them is slowly drained away until all that is left is a soulless wretch, a Dreameaten horror good only for menial labor and shock troops.
How do the raksha operate? One of two ways, they either show up in force and destroy everything and take what they want; which is bad. But far worse is when they show up in the forms of beings you would otherwise trust. They gift you with 'forbidden' knowledges and blessings that turn out to be curses. You will find your kingdom flourishing under their tutelage until one day everything crumbles into ashes and all that is left wandering your streets are madmen and the 'demons' or 'gods' that brought you low.
Where do the raksha come from? Chaos, the primordial sea which existed before their was a Creation. Before there was Creation, there was chaos. And with Creation, came disparity. Life and death, and all that.
And what does Chaos do to Creation? Well, you can steal bits of Chaos and use it to fuel Creation. The world is an engine, designed to burn Chaos and fuel itself to run its engine of wonders. But if Chaos infected the engine, the engine would break down, stop working. One by one, the sun and stars would go out. The barrier between life and death would cease to function. Time itself would run down.
So if you're looking for your Dark Souls in Stygia you are looking in the wrong place. Look outward, to the smiling raksha offering freedom from disparity.