Darksouls are beings from the Plane of Horror, who have come to this world to feed on the life-energy here.
Darksouls are the hungry souls of those who have done great evil, given a new horrific existence by their dread and unknown masters.
Darksouls are pieces of the Shadow-plane ripped from its bearings and stranded in our world.
Darksouls are the images of evil left behind when one turns away from a mirror.
Darksouls are the extremities of a single vast, blind and mindless parasite that lives below the plane of our world, merely the feeding tubes it sends into the world of life and energy we know.
Just as sunlight can be trapped in rocks and plants to leak out in the dark, so Darkness itself can be trapped to take the form of Darksouls, even in the light.
No-one knows what Darksouls are. They just know what they do. They feed, they feed off the life-force of people who go into the Dark.
They look like shadows. But they're shadows cast by no visible light, without a body to be the source of the shadow.
Sometimes, when they're attacking, they look a little like people. They seem to have limbs and a head atop a vague swirling body-shape as if of a robed cowled figure.
Is that because people understand them as people-shaped?
Or is it because the Darksouls themselves will mimic their prey?
What do the shadows look like to dogs or mules or giant spiders? Do they appear to be humanoid? Or does the Darksoul project an image back into the mind of the being it is hunting?
Are Darksouls actually intelligent at all? Or are they merely drawn to living flesh as water is drawn to flow downhill, blindly hunting for the life-force they dimly sense?
No-one knows. No-one has ever interrogated a living Darksoul, no-one has ever examined a dead Darksoul, no-one has ever captured a Darksoul. They have killed Darksouls, or been killed by them, or fled very far away. Darksouls cannot be reasoned or bargained with, they cannot be distracted or amazed, they cannot be confused. They are as implacable as Winter or Age or Gravity. They can only be fought or outrun.
In D&D, Shadows are not undead. They are creatures made of Darkness. Their origin is mysterious and they are - or should be - terrifying. Many years ago, in 'tidying up' D&D (cutting a lot of extraneous and 'silly' monsters, generally being quite 'logical' about it all), I decided that Shadows were Undead - somewhere around Wights, I think, on the Turning Table. Years later, I found out that AD&D had done the same.
Obviously I was wrong. Shadows are not just the souls of evil people or those who have died in torment or the result of a particular necromantic spell. They are Manifest Darkness itself.
And I've randomly generated a lot of them on Level 3 of my megadungeon. I think there will be some sort of portal there. Not too - that would be too horrifically deadly - but at the very least from. A wound in the fabric of reality through which the Darksouls seep from the Plane of Horror, always and forever seeking to feed.
That'll be nice for a party, eh?