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This tale is written in Terza Rima, a rhyming patter set in aba-bcb-cdc-ded, a pattern originating from Dante’s Divine Comedy.
It's not my best work, and I could do creepier, but this was one I just felt had to be written. Kept me up till 2:30 AM to do so, the b*****d.

Caspian

I have seen a thousand horrors, my time on this Earth.
Of all of which, be only one, that I say I abhor.
It came to us, the night my love did ever first give birth.

I took one look, one glance, and shook, and whispered “Nevermore.”
It was a thing, it breathed, it listened, saw, and stared at me.
That stare, nay, not a stare, a glare, it shook me to my core.

And then I realized, in that moment, what I had set free.
The sin himself was my first thought, this creature in the room.
‘Lo, I was wrong, at once I knew, this being was not he.

Yet that did little, nothing, to dispel this hollow doom.
We took it home, we gave it name, and food, and board, and bed.
I thought at once, we let it in; this place shall be our tomb.

On that first night, I thought, I knew, it saw into my head.
I lay awake, beneath my sheets, and stared into the black.
I thought of Caspian, the son, the living boy born dead.

And as I fret, I swore I caught the darkness staring back.
With eyes of evil, worse than death, a terror without age.
I realized in a soulborn gasp, just what the child lack.

Just what the thing was in that room, within that fleshbone cage.
But through my thoughts, their pierced a sound, of which I’ve no description.
Mingled with the creak of wood, footsteps that groaned with rage.

At once, I knew, he’d seen my mind, this walking contradiction.
I breathed in deeply, stood upright, and crossed the room in strides.
For I knew Caspian was coming, bearing his affliction.

I cannot say in honesty, my thoughts were of my bride.
But, in truth, my first concerns were me, myself, and I.
Afeared, within my bones and blood, to see the other side.

The footsteps met the stairs, and as they rose, I wondered why.
Why was I the only one who saw the creature true?
The doctors, nor my wife, did see, my claims they would deny.

When I brought his eyes to question, they ignored the hellish hue.
My mind it kept to racing, as I searched to arm myself.
I thought of those, that did ignore, kept on without a clue.

How could they ignore the ears, in points, much like an elf.
The steps and sound grew closer, as they ambled down the hall.
And my mind, it focused on a knife upon the shelf.

As I grasped the sterling handle, the thing let out a call.
And I was reminded of the thing that it had not.
This creature, beast, it bare no soul, it had no soul at all.

I sidled up next to the door, my conscious burning hot.
I knew, that when it stepped inside, that I would take its life.
I would stab it, burn it, toss it in the ground to rot.

The sound, it came; the door it opened and I swung the knife.
My weapon swung true in an arc as flesh and bone met blade.
And all at once, I came aware the screaming of my wife.

The thing looked up into my eyes, and quietly, it bade:
“Why do you kill me? ‘Twas your soul, that I had come to give.”
And all at once, I fell to tears; to my knees, where he laid.

Realizing then, the thing that I deny the right to live.
The sound he made, was one of truth, I thought between my cries.
He’d held my soul, and it was I, who was deformative.
And had come here to return it to me; that I realize.
Is it murder-suicide, if only one man dies?

No. But you're close. The man's son was born without a soul, and thus, his soul went into his child. The entire thing is him coming to terms with his new soulless existence, filled with spite for the thing he gave it to.

Also, if you like the pattern, read Dante's Inferno. That's where I got it from.