Anon here: I watched this video (https://youtu.be/BL1cf2snegQ) the other day about an alternate universe in mlp where there’s this infection, the divine swamp fever, that’s turning ponies into zombies which then turn into trees. Practically, it's scaryface horror misery porn that I shouldn't take too seriously, but seeing these cute characters suffer activated my saviorfag complex, prompting me to do a bit of writing. I really don't know anything about this franchise besides what's in the video and a few characters' names, so don't expect ironclad fidelity to the source material. The narration is pretty rambling and inconsistent also since I was just getting my thoughts onto paper one night, not taking it too seriously (until about the halfway mark at least). I just wanted to write a touching, albeit kind of masturbatory, story about self-sacrifice and its emotional ramifications. Enjoy:

To set the stage, my self-insert pony is in the hospital with the others researching and trying to find a cure for the disease. As some of the evidence suggests, there is a strange psychological component to the infection, in that it is suspected to spread mimetically (i.e. just looking at or thinking about the disease might cause an infection). Furthermore, some of the infected ponies’ dreams suggest that a metaphysical entity is behind the infection itself, manipulating reality in order to assimilate ponies to its hive mind.
Like I said, misery porn in that the problem is so bad that there’s no hope and no cure since it’s an interdimensional god that’s waging war on reality and everyone must suffer because there’s no solution. Not on my watch. So my original character starts to get hip to this situation, and quickly realizes that we must change our tactics away from fighting-off an epidemic to fighting a war for our survival.
"The target is no longer the tiny spore that’s making us sick, it’s whatever entity is anchoring itself to our reality through the tree of harmony. We’re going to take the fight to that tree, shock it out of its hiding, and make it leave (pun not intended)."
"However, this will never happen if we’re still susceptible to its attack. Our brains are its point of invasion. With my expertise in neuroscience, I am going to perform a lobotomy on myself that will, with hope, render my psyche immune from its assault."

"The price that I will pay for this procedure is a serious one—the pony that comes out on the other side of it will not be the same one that you all have known. I will be a shell of my former self, entirely devoid of my personality, sentimentality, and most memories. All that will remain is a survival instinct, a degree of wit, and the ruthless urge to fulfill whatever purpose that I leave for myself (e.g. I’ll write a note to myself explaining that I need to stop the disease and the plan to go about it). With this, I lay down my life for my friends—a pony’s highest honor."
I brief the ruling board on my plan and secure their approval for a small, elite task force that will accompany me on my mission. Whatever the order of events, my lobotomized self sets-out as team leader to the tree of harmony. P.S. should we fail to stop this plague, I have left the instructions for how to perform the lobotomy, in hopes that it might be used to save others from infection as a last resort.

My team has made it to the tree of harmony. Whether the culprit is the tree itself or whatever entity that might have possessed it, this spot seems to be the focal point of our problems. Because of this close relationship between the tree and the infection, the strategy is to shock the tree using a series of bore-hole explosives to demolish parts of its body. Simultaneously, pegasi will assault its leaves by slashing and chopping its branches. If my theory is correct, then this unexpected blow to this thing will have a positive effect on the infection for us—if the tree is too busy reeling from its injury, the progression of the disease should halt in its tracks (we will try to confirm this outcome via relay with the hospital).
This, in and of itself, would constitute a victory (temporarily halting the disease until the tree recuperates), and potentially reveal a victory condition (utterly destroying the tree to permanently stop the disease. However, I am after more. Whatever is behind this disease is intelligent. We can negotiate with it—perhaps even coerce it. I want a better outcome than “just ok”.
The explosives blew chunks of tissue out of the tree and a great number of its boughs fell to the ground. Braindead as I may be, I left myself instructions on what to say and how to act here. I threaten the entity with further damage unless it reveals itself, warps reality to reverse the disease (which I’m assuming is the mechanism behind all of the anomalous and logic-defying ways that this disease spreads and progresses), and leaves this dimension forever.
To the amazement of my team, a physical manifestation emerges from one of the gaping wounds in the tree; but it is not so compliant. It swells its power in defiance and rapidly infects every pony on my team. In just seconds, they begin convulsing and vomiting that fetid black bile as the entity descends closer and closer, seemingly content in its achievement of subjugating those who defied it.
With quick thinking, however, my pony realizes the predicament and hatches a plan. A risky gambit, but this being must regard ponies with some degree of importance (enough importance to wage war against our race and civilization), so I think it will pay off. I start to fake convulsions and fits of coughing as well, mimicking my comrades’ symptoms to the best of my ability and obscuring my act as from the still approaching entity much as possible…just a little…closer… … … there!

A knife sprung from my pocket the second that it was withing striking distance. My foreleg now wraps around the entity, wrestling it to the dirt while the implement held in my other hoof begins to pierce it. As suspected, the shock of the moment has halted the progression of my team’s infections. “Reverse.” I stoically demand, increasing the force of my blade. “Reverse. Reverse. Reverse.” Over and over and over again until my team’s symptoms begin to regress. “Reverse. Reverse. Reverse.” Over and over and over again until relay from my comrades confirmed similar good news from back home. “Reverse. Reverse. Reverse.” Over and over and over again until I was sure that all the ponies that I once loved were safe again.
Once this feeling came, my orders changed. “Leave. Leave. Leave.” As I slowly released the entity from my subjugation. “Leave. Leave. Leave.” As I kept feinting swipes and stabs, narrowly and deliberately missing its form. “Leave. Leave. Leave.” Until I was certain that the fear that I had struck into it was enough for it to never return until the end of time, and it would never be brazen enough to make any pony suffer again, lest it experience my being once more. “Leave. Leave. Leave.” Until I was finally slashing at nothing but air…finally.
The war had been won. My elated team cheered harder than they thought that they ever could. Despite my elevated breathing and heart rate, I remained silent—it was all I was able to convey. I had fulfilled my task. That is all.

The heroes return home, met with the equal exuberance of the hospital staff who remained back. Once more, no cheers nor emotion left my lips.
“Aren’t you happy?” Twilight queried.
“I fulfilled my task.” Was the response that she was given.
After an additional short moment of meeting her concerned gaze with my own vacant stare, I slowly began to leave the room for my own quarters. There was something that was left there for me.
As I left through the doorway, Twilight, stymieing a few tears, remarked, “He’s…he’s…really-” Twilight paused, “I’m really going to miss him…” (it’s my fanfic, so I’m gonna say that we were close acquaintances, ok? The story needs to have emotional stakes and ramifications.)
“Yes, but this is what he wanted. Success or failure, he knew he wasn’t coming back and he was ok with making that sacrifice for us.” Princess celestia responded (or maybe it was the black princess I don’t really know these characters tbh)

They followed me to my quarters to see me retrieve a note from a small box on the floor. This note was the last remnant of the pony I once was.
Its instructions were simple: “Find Pinkie Pie. Protect her. Obey her. Love her.” I read them aloud.
From here until the end of my life, this was my new purpose—the final will of what I once was.
“Where is Pinkie Pie?” I turned my head back to ask in my monotonous voice.
“Search and rescue parties are already organized and working to retrieve ponies who were lost in the woods during their infections. If we are lucky, she is already back in Ponyville waiting for you.” Twilight responded, matching my tone as best she could so as not to stimulate her emotions any more.
Rarity was there too I guess.

In the forest, Pinkie Pie was found, somewhat shaken and with rather muddied memories about what she had endured, but ultimately safe and eager to return home.
“Pinkie, we heard that your lover was the reason that we were cured. I’m sure he’s waiting for you back home!” One of the rescue members informed her.
Excitedly, she ran all the way back to town. I arrived and was waiting for her, instilled with the same purpose—Find Pinkie Pie. Protect her. Obey her. Love her.
The first task was easy as she sprinted over to embrace and kiss me.
“I was told that you are the one who saved everypony, is that true, my love?” Pinkie asked once her excitement found a momentary break.
“I fulfilled my task.” Was all that I could produce.
Twilight interjected to quell the confused expression that was beginning to brew on Pinkie’s face. “Yes, he did. But he made a great sacrifice in order to do so. To immunize himself against the infection, he had to deliberately destroy part of his brain. It was the only way. He has no personality or sentimentality anymore.”
Pinkie’s expression began to sadden and her breaths stuttered in disbelief.
“But, Pinkie.” Twilight began to protest, “The part of him that remains is the part that loves you. The final instructions that he left for himself were about you.”

Pinkie turned to meet my vacant expression again. I handed her a note. This note contained the final words that she would ever hear from me.
She began to read aloud, “My dearest Pinkie, I am about to undertake the heaviest burden that has ever been entrusted to me. From whatever combination of circumstances, I believe that I am the only one who has a chance at putting an end to all this suffering. I have to do this. I won’t ever be able to see your beaming smile again, but know that my desire to see it everyday is what animates me. With hope, it will continue to do so for whatever remains of me after I have fulfilled my duty. This remnant, I am leaving for you. Until the day I die, I will obey you. I will protect you. I will love you, in return for the joy and love that your presence has shown into my life all the while we were together.
I love you so much. My only selfish request is that you continue to remain by my side, even if I may never be able to match your smile again. Just as you were when I was alive, you remain my one love and my one purpose until the day I will die. Please continue to show me your smile. Even as I write this, it is my sole comfort knowing that you may show your happiness once more if I am successful. That is why, although my mind must leave you, my body will forever be by your side. Yours truly, self-insert OC.”

The tears were streaking from Pinkie’s eyes long before she read the final words. Neither could any of those around who been moved by the display of love and sacrifice contain their composure. Pinkie once more leapt forwards to wrap her forelegs around me in the tightest embrace I had ever known.
“Crying. Are you safe?” I asked, with reflexive concern.
Pinkie could barely keep enough breath between her sobs to respond, “Yes. Yes. As long as you are here!”
“I will always be with you. I love you.” Were the words that she was met with. Despite the monotonous tone, all could detect the sincerity.
“I love you so much too!” Pinkie wailed. Eventually, her bellows lapsed into laughter as she became contented with the situation. “I’m so happy that you loved me, and you’ll always be mine because of it! I’ll be yours too!” Pinkie once again exclaimed and sobbed.

One last part of the note that she had missed in her excitement, however. She read silently for a moment, before springing into the air. She tossed the note in Twilight’s direction.
“Read it! Read it! Read the bottom!” Pinkie gasped.
Twilight began to quote the final words that I would even pen, “P.S. In the box beneath my bed, there is something that belongs to you, if you’ll accept me.”
“I think I know what it is! Let’s go! Let’s go!” Pinkie exclaimed once more before bolting off, followed closely and intently by her lover and then by the rest of her friends in company.
When reaching the foretold box, Pinkie found the ornate betrothal band that was meant for her, evidence of her lover’s final wish. Despite the circumstances that she found herself in, she couldn’t be happier at the outcome, and, whatever thoughts lay behind her lover’s eyes in that moment, he must have felt some kind of happiness too…

…

“To lay down one’s life for their friends—that is a pony’s highest honor.” Quoted the statue commemorating the brave heroes who faced and subdued the divine swamp fever.
“It looks like me.” I said. “Why?”
“Because of the sacrifice that you made to save us, my dear.” Pinkie tried to remind me.
“I fulfilled my purpose.” I responded apathetically, in what must have been a proud acknowledgement of my deeds. I continued to gaze at the effigy.
Although many months had passed since he had been able to show emotion, his friends swore that the feintest curl began to show itself upon the edges of his lips. Was it just a muscle twitch? Who can truly say?

Fin.