This plot bunny was horribly persistent. Spoilers for Skyward Sword. Note: this takes place during the Child Timeline (if you don't know what that it, just google Zelda Timeline) and after Four Swords Adventures.
The only warning is that there are hints at Ghirahim/Demise and/or Ghirahim/Ganondorf, but it can be taken as insanity as well or lust for power.

But as a brief explanation: Ghirahim didn't die in Skyward Sword, instead he becomes Dark Link's Sword whenever Dark Link is summoned. (Basing upon the idea that I have seen before Skyward Sword: If Link's sword is the bane of all evil, then is Dark Link's sword the bane of all good? and upon the idea that Ghirahim is the anti-Master Sword)

How many years have gone by?

Really, Fi's choice of going into an eternal sleep was for the best. I was going crazy from all of these reincarnations, of all of these losing battles, and of never being held by my proper master!

Although it really has only been three battles… three times that the soul of the Goddess's Chosen has been made to fight my true master. (At least in this timeline; I know there is at least once more with the crazy time-traveling that confused me for a few centuries until the next battle.)

Yes, I know he is only an incarnation of my original master's hatred, but that means he still is a part of my master. But I've never seen him, actually I've never seen anybody since that fated day that my master was defeated. Thankfully time has allowed me to get past that once trembling, mind-addling, blinding rage and has turned it into a minor irritation. But that doesn't mean that I don't hold a grudge.

Oh and how I hold that grudge!

That was why that the next time there was a battle I would twist things around a bit.

Mind you the wait was so droll! I had to wait until seals were weakened – or broken – and then until my master had summoned a dark clone of the Goddess's Chosen's incarnate.

Yes, I was Dark Link's (or Shadow Link as he was more recently called) sword. My form felt too slender, it felt too much like the shape of the Master Sword. I didn't like it. My power was diminished in this form, and whenever I was not being held by the polar opposite of the Chosen Incarnate I was sealed within a pedestal of the Dark World.

Well, it hadn't always been that way – before I was just scattered within the winds, my mind separated from my body. But then my master's hatred had summoned a creature – one that would rival Link in every way. And I suppose that the complete contrast of the Chosen One combined with a semblance of my master's powers was what had called me once more to battle. What better blade to use than the once that was the opposition of the Master Sword?

Sadly, the Hatred did not see it that way. All that he saw was a sword that was similar in shape to the Master Sword, a sword in contrasting colours. He did not see my worth, and perhaps that is the reason he lost.

When Dark Link was defeated I had once again disintegrated – but instead of going back to my previous state I found myself within a damn rock, within the Dark World that the Hatred had created. And because of the fluctuations in time, even when it was turned back, I found myself stuck only being released into Dark Link's grasp whenever he was created.

But this time, I would ensure that my master saw my worth, I would win this battle, and hopefully every other battle after this.

I would fulfil Demise's final wish. I would let his hatred rule over the land and crush the Goddess and her warrior.

.+++.

"Arise my warrior." I heard that familiar voice say. Many a time I had compared his voice to Demise, and while yes it had similarities, it would never be the same. Just as he had compared Goddess to her Sky Child incarnation, I will make the same comparison about his Hatred. Becoming a mortal changes things, it dilutes everything. While his Hatred had charisma, while he had control, it was nowhere near the level that Demise had.

I could feel when Dark Link moved and he did so as he stood from a lower position – kneeling maybe?

He is just going to send you on a mission, one which you will eventually fail. One which your light will conquer you over. Pathetic, isn't it Dark Link? Such a meaningless life. I telegraphed into his mind.

From the frantic movements my sheath – and I – had done I presumed that Dark Link had glanced about.

"Who's there!"

I'm in your head you idiot! Don't cause concern for Master. I wouldn't like it very much if you were sent to oblivion and I to the Dark World. It's quite boring there. How about you give me to Master?

"I'm not giving my head over…" Dark Link mumbled.

I thought out a sigh. I'm the sword. Give me to Master.

"Dark Link… is there a problem."

Uhoh… he's angry. And don't you answer aloud. Trust me, if you do and you say the words I think you will, he won't hesitate.

"Uhh…" Dark Link fumbled for words and I felt my sheath moving – being unhooked and held horizontally in his hands. "The sword says I should give it to you."

I am not a thing!

"What the hell should I call you then?"

I heard the Hatred give a cough.

"Sorry, it was talking to me…"

"A talking sword? Interesting…"

The moment his fingers brushed my hilt, ecstasy filled me. Oh! Master! I could feel the power in his touch; I could feel Demise flowing from deep within him! I wasn't even being held or grasp, but just that touch! It was almost too much! It has been far too long since I was last in the hands of my rightful owner! How I awaited for this day to come!

Master…

Yet those fingers left as quick as they came and they took away that delicious dark magic and my joy along with them, leaving me hollow.

No! I despaired. Don't reject me Master! No! No! I'll be of worth to you! Just give me a chance!

"Will you please shut up blade?" Dark Link complained. "Oh, he's whining."

I am not whining brat! Watch your tongue!

Pompous bastard! I heard Dark Link accidentally telepath to me.

Arrogant mortal! I returned. Don't you get it into your head that you're better than me, or that I am your servant!

"But you're my…"

I am not your sword!

Dark Link recoiled from my mental shout. But that was good, he was to fear me, he was not my master, he would never be my master! Such audacity! It made me furious! I only belonged to Demise, I only belonged to his Hatred!

"Hm… the sword didn't do this last time…" I heard the Hatred thoughtfully say.

"Glow, or talk?" Dark Link asked quietly.

"Both."

The moment after that one word was said my world exploded. Where there was nothing there was now a hand, where I was hollow, I had been filled with the power of a God. My mind was white at the sheer pleasure of being held by him once again! Nothing else was as significant as this moment!

"The sword is quite silent. Are you sure they were talking?"

Ah… yes, if I wanted to continue being held I should give him proof, I realized as I gained some semblance of thought after my high.

If he is crazy, it is no doing of mine Master. I sent to him – a thrill coursing through me.

I heard his chuckle through our telepathic link and I had to strain to remain coherent. So you really are a talking sword. Why do you chose to call me master?

It was not my choice to make Master, it was you who chose me after I proved myself to be of your worth.

I do not recall such a memory Sword. The Hatred sent to me. Yes, it was only him who understood right away of our telepathic link, it as only him who could send such excitement through me from only his grip.

My apologies Master, I have not explained properly. I am the chosen sword of the God whose hatred you have been reborn from.

So… The Hatred's telepath was dark, it was evil, it was intrigued. I am a God reborn…

I know it is not my place to correct you and it is my fault for not elaborating, but that is not quite so. Most of his essence has been sealed within the Master Sword – just as you were sealed within the Four Sword – but his hatred lives within you. However, perhaps you may be able to absorb the remainder of him at a later time.

Tell me Sword, what type of God deserves such a treatment? What kind of God creates such a hatred that it must be reborn?

My mind was slipping back into the emotions, the bliss was corroding the state of consciousness that I was struggling to maintain, but I had to at least answer this one question. The one that arose from the demons, the one who opposed the Goddess Hylia. His voice, his power, his hand… my control was slipping again and I knew why. It had been too long since I had last felt it – felt that powerful grip – and I had forgotten how to overcome the mind-addling emotions, how to conquer it. I hope you do not mind me asking, but can I returned to my demonic state Master? Conversing with you this was is rather… taxing.

So you are not just a blade… very well Sword, how is this done?

Well, first I should be returned to my original stare – I fear what I would turn out as in my current state. Just will me blade to return to its natural state before willing be to turn into the demonic sheath. My coherency was blurring into the sea of bliss once more, and I didn't know if I could return to converse with him this way until I had accustomed to his grip again.

"Very well." He spoke aloud before his power surged through me, creating an all new high as my own power was restored. I didn't hear his reaction – only feeling as he tested out my restored blade. But soon I could feel my consciousness becoming separated, limbs forming and movement enabled.

I was once again my demonic, invincible self and I resisted the urge to laugh and open my eyes. Instead I stretched out my arms and left my most vulnerable stop out. "Stab the diamond with the blade Master." I spoke – pleaded.

The shifting of his grip on my hilt was distracting enough that I was almost thrown off balance as he swung the blade and stabbed me.

"A-Ahn!" I moaned out and faced the sky as both parts of me were joined. When it was finished I let out a soft breath and opened my eyes for the first time in millennia. I ignored the vast differences in the land and instead concentrated on the two people before me. To my left there was a man of shadowed colours, and a man who looked too much like that meddlesome, gadfly, Sky Child. "Tch, at least I know who to look for!" I said in distaste before turning to the man on my right.

Yes… there was no doubt… "Master." I said and fell to one knee and folded an arm across my chest.

"Arise my sword."

With a cunning, plotting smirk I did just that. "Yes Master Ganondorf. Ghirahim – Lord of the Demons – awaits your orders."

:Edit: If I get enough requests, I MAY continue it (No promises though, I have a lot of multi-chaptered fanfictions going on right now)