Eternal

Many years after sending her beloved protege to Ponyville, unease sits heavily on the heart of Equestria's Sun Princess, as her relationship with Twilight Sparkle seems to have cooled despite all they've accomplished together. On the advice of her sister, Celestia sets out to investigate and resolve the lost connection with her most faithful student.

A Fanfic for My Little Pony: Friendship Is Magic by device heretic, Eternal explores the bond between the Sun Princess and her Faithful Student... as well as what, exactly, both those titles truly mean.

Tropes used in Eternal include:

Anthropomorphic Ponification: The Sun takes on the form of a radiantly cruel Celestia.
- On top of this, Celestia is 'the part of The Sun that's a pony', while The Sun is the part of Celestia that is the sun.
Arc Words: "Eternal Sun" is more than just one of Celestia's titles.
Anchored Ship: Twilight and Celestia can't express their feelings.
Break the Cutie: The war with Discord left serious psychological scars on Celestia. The Sun was also innocent, naive and trusting until Celestia snapped.
Broken Ace: Twilight is striving to become a Type 2. Celestia already is.
Broken Pedestal: Celestia fears becoming this in Twilight's eyes; Princess Luna also fears breaking hers even further by revealing more of her past to Twilight and her friends.
Dream Weaver: Luna has this ability to some extent, as dreams are one of her domains.
Driving Question: What does it mean to be the "Faithful Student"?
Earn Your Happy Ending: The ending is very upbeat, especially for Celestia, Twilight and Luna, but it took a lot of hardship to get there.
Green-Eyed Monster: Princess Luna eventually admits they allowed the situation to deteriorate out of jealousy over Celestia and Twilight's formerly close relationship.
- Twisted around with the minor plotline of Dash and Scoots' fighting: a large part of the problem is that Dash secretly resents Scootaloo for becoming a better Captain than she was, when joining the Wonderbolts was always her dream.
The Guards Must Be Crazy: Averted when Twilight's friends first try to disobey Luna's orders and get past the guards.
I Did What I Had to Do: Celestia's justification for chaining The Sun.
I Hate Past Me: While viewing a vision of the past, The Sun Itself is visibly irritated by seeing their past self being friendly with Celestia.
I Know Your True Name: Twilight figures out the true identity of Nightmare Celestia, giving her insight on how to defeat them. It's Guilt.
- Also important elsewhere in the dreams, especially when Nightmare MOON ends up coming in. Her true name is Sister. Or when dealing with herself and her own identity.
Irony: Celestia sent Twilight to learn about and report on the nature of Friendship, yet both struggle to put those lessons into practice with each other.
- The Sun points out another major Situational Irony: as the part of The Sun that is a pony, Celestia was always meant to live like one, experiencing all the world had to offer in ways The Sun could not. Instead, she became so obssessed with being the perfect Eternal Sun Princess that she grew distant and detached, the exact opposite of what she was meant and wanted to do.
Journey to the Center of the Mind: Used to get to the heart of Celestia's problems.
Last-Episode New Character: Radiant Dawn
Magical Underpinnings of Reality: The ramifications of Celestia's role as the Sun Princess are explored.
The Mare Wearing the Sun Princess' Mask
- Becoming the Mask as history unfolds. Breaking past the masks is an important theme.
Martyr Without a Cause:

Luna: You're just like her. You've both turned self-sacrifice into a vanity.

Not So Different: Twilight and Celestia.
Official Couple: Fluttershy and Big Macintosh.
The Perfectionist: Twilight, much to Celestia's alarm. Celestia as well.
Plot Parallel: Rainbow Dash and Scootaloo are having serious issues with one another due to Scootaloo becoming the Wonderbolt's captain after Dash's early retirement. Eventually, Twilight comes to realize how this mirrors her own problems with Celestia.
Poor Communication Kills: The tragedy of miscommunication is a recurring theme.
Promethean Punishment: The Sun ultimately forces Celestia to cycle between being Princess of the Sun and living a normal, mortal life. Celestia isn't quite sure if this falls under an Unishment or not.
Ridiculously Successful Future Self: Since Eternal is set several years after the original series, a few of the Mane Cast have hit this.
Secret Test: Played heartbreakingly early on, when Celestia realizes Twilight sees her friend's wedding as one of these and is desperately trying to impress her by ensuring everything's perfect.
Ship Tease: A running theme early on is Celestia's growing confusion about what, exactly, Twilight means to her, and vice versa. Rainbow Dash and Spitfire get plenty of their own teasing later on.
This Is Something She's Got To Do Herself: Though fully aware of how The Power of Friendship has seen her through in the past, Twilight repeatedly wavers at the thought of getting the others involved, and eventually resolves to do this. Both Luna and Spike argue the point using this reasoning.
Tragic Dream: Twilight expresses her unfulfilled desires for Celestia by sinking into dreams.
Tragic Mistake: As Twilight eventually discovers, Celestia's tragic mistake was binding The Sun to avoid setting, and convincing herself the ends justified the means.
Wanting Is Better Than Having: While all of the Bearers achieved their dreams, one of them explictedly discovered this: Rainbow Dash found leading the Wonderbolts was too restrictive and demanding for her tastes, and wound up leaving the position.
Was It Really Worth It?: Celestia asked herself this after banishing Luna.
We Used to Be Friends: Celestia used to be close to The Sun, before her betrayal.
Wedding Day: Fluttershy and Big Macintosh are getting married, giving Celestia an excuse to visit and see precisely how Twilight is doing.
What the Hell, Hero?: Somepony has to answer to this.
A Wizard Did It: A Running Gag is Twilight's increasing frustration whenever she's informed that something has a mystical origin too complicated to explain concisely.
You Don't Want to Know: When Celestia tell Twilight Sparkle that she is the part of the Sun that is a pony and the Sun is the part of her that is the sun, Celestia immediately says that she won't explain it as the answer is something Twilight won't like. Twilight Sparkle, however, merely wanted to know if Luna had a similar arrangement.
Your Mind Makes It Real: While dreaming, they still can still experience severe pain. Luna attacked by Twilight's mental defenses. Twilight bound by gold chains. It also works outside the dream. Luna ends up scared when, after smashing through a glass table in her dream, Twilight's real body has slashes across it.