...Welcome my friend... Welcome to Eternity...

Here, you have nothing to fear, nothing to hide from, nothing to be scared of...

Except... Eternity...

At first Twilight thought that being an Alicorn was amazing, but then her friends continue to age around her as she stays the same.

Twilight Sparkle is successfully fulfilling her destiny as the ruler of a kingdom somewhere. Although she has continued to write to Princess Celestia consistently, the latter has distanced herself in recent years. Twilight writes one final letter to her teacher.
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Personally, I prefer the first chapter on its own, but the story does continue in the rest.
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Part 4 in progress!

Twilight has successfully restored her friends' cutie marks to normal. But something still remains to be fixed.

Starswirl the Bearded's spell.

But something happens to Twilight... Something that makes her friends prove what they would do for her.

Based off of Black Gryph0n and Bassik's song: Another End. The final song in their IMmortal album. The link isn't for a YouTube Video because they aren't all there. The link is to their bandcamp page where you can listen to all the songs in the album.

Changed from the Unicorn of old, Twilight Sparkle comes to terms with being the ever esteemed Alicorn as granted by her completion of a new spell passed down by Star Swirl himself. However not all is sitting tight quite yet. A nagging feeling plagues Twilight, about her own mortality.

http://gashi-gashi.deviantart.com/art/Twi-351500122

Tonight, I feel like destroying something beautiful.

Those were the last words spoken to me by my faithful student.

Time has come once again for Princess Twilight Sparkle to band together with her friends and save Equestria from another threat looming on the horizon. However, when she sends for help from the Element of Harmony, she gets a response she'd never even considered before:

No.

Months have passed since Twilight became a princess, and yet she still can not bring herself to accept her new role. That's when she starts thinking. What would happen if she quit?

After Twilight's psychotic break involving the Want It, Need It spell, Princess Celestia banishes her former student from Canterlot Castle and severs all ties.

A young and emotionally tortured Twilight Sparkle refuses to face facts. The same insanity that pushed her to the brink when she made that unforgivable error in judgment now compels her to fix it. Twilight will stop at nothing to get back in Celestia's good graces.

No, that's not right. She won't stop there, either.

Written for The Writeoff Association's "Princess Not Included*" contest.

Twilight's relationship with Celestia becomes fragile after Twilight learns that becoming an alicorn came with a longer lifespan, meaning she would outlive all her friends. Then her friends end up in the hospital under mysterious circumstances, will they survive? And will Twilight's relationship with Celestia?

Authors Notes: I appreciate viewer discretion and respecting of my opinions. People tried to convince me that immortality is a good thing here, but I am not a believer in that and never will be. To counter the comment that started that little debate, it's a fact that the highest age for suicide is people in their fifties. Some can overcome the pain of losing loved ones, others are overwhelmed by sadness.