I'm Here to Help

Crystal Tokyo.
Crystalline structures, delicate spires of magic reaching to the heavens like angel's wings.
People laughed, people smiled, people almost never cried. Why would they? They lived in Utopia.
I hate it. I hate them. I hate her.
Serenity. 'Neo-Queen' Serenity.

I'm Here to Help is a Sailor Moon fanfic mostly written in 1998 (but finished in 2007) by Mark Doherty. What seems to be a typical "Big Bad wants to change history" plot upon first reading quickly becomes much, MUCH more. Notable for being one of the first and best written Crystal Tokyo As Dystopia stories, which many have since tried to match.

Tropes used in I'm Here to Help include:

Assimilation Plot: What the "purification" did to the majority of mankind. The few that were strong enough to reject it or otherwise weren't affected are regarded as enemies by the Senshi.
Badass Boast
Bad Future
Beware the Superman: Emerald asked why the Senshi should rule the world just because they had powers instead of making it better in one of his diary entries.
Big Bad: Neo-Queen Serenity, and the (future) Senshi. To Emerald, Beryl and even Metallia are not worth the time.
Bittersweet Ending: Emerald never gets to fully enjoy his victory as he's erased from time moments later, but his younger self will not have to go through the pain he did.
Blatant Lies - Emerald's "past lover" story. Also the lie to Queen Beryl that he served under Jadeite, but she doesn't believe him.
Blessed with Suck: Emerald's view of immortality, after many years of suffering and fighting.
Brainwashed: Everyone in Crystal Tokyo and the rest of the Earth, except Emerald and those exiled. The result of an Assimilation Plot, and the main reason Emerald wants to change the past.
Clark Kenting: It takes Emerald a long time to figure out Sailor Moon and Serenity are the same.
Deconstruction Fic
Dramatic Irony: The protagonist is unaware that Sailor Moon is Queen Serenity.
Energy Absorption: Emerald needs to do this to get stronger, mostly by draining youma to the point where they die and turn to dust.
Exact Words: Sailor Pluto sees Crystal Tokyo prevented from coming into being as it existed through no act of her own. Meaning by arranging events so Emerald be forced to go through with his fallback anti-Purification plan, and being in the right place so he could defeat her and feed off her energy, Pluto could prevent the future she didn't want without breaking her oath to Queen Serenity.
Fashion Victim Villain: Emerald described the Senshi as "girls with interesting powers in silly suits. As opposed, of course, to women with horrifically intense powers... in silly suits."
Fate Worse Than Death: Being conscious and aware for centuries frozen in ice, as Emerald was. Also Beryl's "Eternal Sleep" punishment.
For Want of a Nail: It is heavily implied that in the original timeline, Emerald would have became Wiseman. Pluto's "If you knew your original fate, you would have thanked me" line cinches it.
Future Badass: Come on, guess who.
Heroic BSOD: After Emerald finds out that Sailor Moon is the Serenity he has been looking for all along.
Identity Amnesia: Emerald cannot remember his own name or much of his past. His true name is revealed to be Yukio, by Pluto.
- Jadeite, after being freed from his Eternal Sleep, remembers everything about his past before his brainwashing by Beryl, leading to a moment of sheer, screaming horror as these memories overwhelm him. Emerald thinks he's simply gone crazy and so snaps his neck.
In the Name of the Moon: Subverted, in that Emerald thinks it's over the top and at one point interrupts Sailor Moon during one such speech.
Knight Templar: All of the Crystal Tokyo-era Senshi, especially Sailor Mars.
One-Hit Kill: Emerald does this to one of Kunzite's youma, with Kunzite himself standing there. And then to Kunzite himself later on, as well as many more Dark Kingdom fodder.
Power Crystal: Emerald's green crystal that he keeps around his neck by a leather chain, though he's quite powerful enough without it. It is also sentient.
Really Seven Hundred Years Old: Emerald is nearly a thousand years old, but appears to be no older than a young adult. Same with all the Crystal Tokyo Senshi, and the King and Queen.
Set Right What Once Went Wrong: The whole motivation of Emerald's life, as written in his journal entries.
Time Travel: From the 30th century, to the time the Senshi first reawakened in the late 20th century (Dark Kingdom Arc).
Unrequited Love: Emerald and Rei in the future, due to them being Arch Enemies on opposing sides trying to kill each other. After journeying to the past, Emerald doesn't understand why he seems so willing at times to aid the younger Rei. May be possibly subverted at the end with the "present" versions of themselves.
What Measure Is a Mook?: From chapter 1:

"I killed it. A creature, youma, demon, call it what you will. Did it have dreams, aspirations, hopes? Did it wish for its life, to be hunted and killed? Did it want to be left alone?"

Xanatos Roulette: At the end, it is revealed that Sailor Pluto was the true mastermind behind the plot to prevent Crystal Tokyo (or at least the "purified" version described) from coming into being by "thousands of manipulations over thousands of years".
- Yes and no. She spent "thousands of manipulations over thousands of years" to bring Crystal Tokyo into being, "Only to realise only after [she] got it that it wasn't what [she] thought it was." After that she started secretly working towards getting Emerald to change things.
You Will Be Spared: Emerald's offer to a horde of youma ready to invade the Earth through a portal... not that they listen despite knowing how strong he really is.