One of the things I like to do while reading fanfiction is to tie in certain aspects of canon and fictional Equestria, be it a single character or race, chapter or an entire story, with one heavy metal song that matches the tone of that aspect describes the same theme with its lyrics. With over 15 million words read so far, obviously there would by many song-story combinations that are worth noting.

A lot of the songs are extreme metal, so I've noted their "Hardness" levels based on this tvtropes scale.

More songs and combinations will be added as I find more. If you have a suggestion, please don't hesitate to comment in here. Exercise caution for listening to any songs above level 8.

[Characters and Musings]

Fluttershy <=> Oceanlab - On A Good Day (Original Mix)

Pinkie Pie <=> Equilibrium - Wurzelbert (Level 8)

Pinkamena Diane Pie <=> Dark Tranquillity - Monochromatic Stains (Level 9)

Rainbow Dash, Equestria's Best Flier <=> Angel Vivaldi - A Mercurian Summer (Level 7)

Rainbow Dash, Struggle Before the Spectral Barrier <=> Periphery -  Racecar (Level 8)

Rainbow Dash to Scootaloo <=> Nightwish - Know Why the Nightingale Sings (Level 7)

Rarity, Against the Run of the Mill <=> Rush - Grand Designs (Level 4)

Rarity, A Sunny Day in Canterlot <=> Do As Infinity - Under the Sun (Level 3)

Twilight Sparkle, the Unyielding Voice of Reason <=> BEING - Arcane Academic (Level 7)

Princess Celestia and Luna, the Conflict (Canon) <=> Tilt Feat. Maria Nayler - Angry Skies (Extra Terrestrial Remix)

Princess Celestia, the First Night without Princess Luna <=> Insomnium - Daughter of the Moon (Level 9)

Princess Celestia, After banishing her sister, witnesses the rebirth of Philomena <=> Eternal Tears of Sorrow - Red Dawn Rising (Level 8)

Princess Luna, the Birth <=> Piotrek Gruszka - Fermi Paradox (Level 6)

Princess Luna, the Bright Side of the Moon <=> Alcest - Écailles De Lune Pt. 1 (Level 4)

Princess Luna, the Dark Side of the Moon <=> Alcest - Écailles De Lune Pt. 2 (Level 7)

Princess Luna, and her transformation to Nightmare Moon <=> Ne Obliviscaris - Tapestry of the Starless Abstract (Level 9)

Princess Luna, One thousand years on the moon, the conflict <=> Solution .45 - Clandestinity Now (Level 8)

Princess Luna, One thousand years on the moon, alone <=> Sybreed - Isolate (Level 8)

Princess Luna, From Moon to Equestria <=> Oceanlab - Satellite (Filo & Perry Remix)

Princess Luna, GamerLuna and her gang <=> Blood Stain Child - Electricity (Level 8)

Princess Luna, with Twilight <=> Insomnium - Down with the Sun (Level 9)

Princess Luna, Keeper of the Dreams <=> Lost Witness Feat. Tiff Lacey - Home (Mike Shiver's Catching Sun Remix)

Changelings <=> Meshuggah - Swarm (Level 10)

=> Changelings, before the advent of Queen Chrysalis <=> Omnium Gatherum - New World Shadows (Level 8)

=> Queen Chrysalis, the Leader <=> Scar Symmetry - The Anomaly (Level 8)

=> Queen Chrysalis, the Usurper <=> Mors Principium Est - Sinners Defeat (Level 10)

=> The Changeling that despises itself <=> Disarmonia Mundi - Venom Leech and the Hands in Rain (Level 8)

Daring Do <=> Nightwish - Wanderlust (Level 7)

Impressions on the Origin of Alicorns <=> alex M.O.R.P.H. and Woody van Eyden meet Talla - Hybrid Harm (Talla Mix)

Eulogy to Unicornia <=> Equilibrium - Nordheim (Level 8)

Equestrian Mock War <=> Equilibrium - Kurzes Epos (Level 7)

Seapony Lyra Trilogy, Part 1: Realizing her heritage <=> Oceanlab - Clear Blue Water (Ferry Corsten Remix)

Seapony Lyra Trilogy, Part 2: Bon Bon by the beach <=> Oceanlab - Sirens of the Sea (Sirens of the Sea (Above and Beyond Club Mix)

Seapony Lyra Trilogy, Part 3: The Distance Between <=> Oceanlab - Beautiful Together (Signum Remix)

Impressions on the Potential Relationship between Spike and Rarity <=> Insomnium - Change of Heart (Level 8)

[Fanfiction]

RagingSemi - Our Last Goodbye (Years 1192~7592) <=> Oceanlab - Sky Falls Down (Armin Van Buuren Remix)

RagingSemi - Our Last Goodbye (Years 10^9~10^40) <=> Christian Zechner & Lira Yin - Starclad (Original Mix)

Flitter - Peter Gabriel - Come Talk To Me

Take the Long Way Home <=> Nell - Eden (Level 3)

Hello, Sedna <=> Starbreaker - Hello, Are You Listening? (Level 6)

The Centerpiece Of My Collection / The Collection Of My Centerpiece <=> Rush - Secret Touch (Level 6)

Arrow 18 Mission Logs: Lone Ranger, A.F.R.T. 11252257:054557  <=> Being - Cosmonaut (Level 7)

Days of Wasp and Spider <=> Sybreed - Orbital (Level 7)

Warmth <=> Kamelot - Serenade

To Love the Changeling Queen <=> Kamelot - The Black Halo (Level 7)

Directive: Grow <=> Scar Symmetry - Holographic Universe (Level 8)

My Little Pony - Hospice <=> Noumena - Prey of the Tempter (Level 8)

The End of Ponies <=> MyGrain - Cataclysm Child (Level 8)

=> The End of Ponies - Revisiting Ponyville <=> Insomnium - Lay the Ghost to Rest (Level 8)

Background Pony <=> Ne Obliviscaris - Forget Not (Level 8)

Background Pony - Chapter XVI, Discord and Princess Aria <=> Ne Obliviscaris - Of Petrichor Weaves Black Noise (Level 8)

The Stranger and Her Friend - Chapter XVI, Celestia banising Discord <=> Eluveitie - Calling The Rain (Level 9)

The Conversion Bureau, Conversion Dreams <=> Widek - Aurora Borealis (Level 6)

The Conversion Bureau (Dark) <=> Scar Symmetry - Radiant Strain (Level 8)

The Conversion Bureau: 27 Ounces - A Cup of Terror <=> Meshuggah - New Milllenium Cyanide Christ (Level 10)

Friendship is Optimal - Celest.AI <=> Scar Symmetry - Orchestrate the Infinite (Level 8)

Friendship is Optimal, Chapter 10 <=> Scar Symmetry - Astronomicon (Level 8)

Friendship is Optimal, Chapter 10 - Hassan Sarbani <=> MyGrain - Xenomorphic (Level 8)

Friendship is Optimal, Chapter 11 - All the Time in the World, Light Sparks <=> Alcest - Faiseurs De Mondes (Level 6)

Friendship is Optimal, Chapter 11 - All the Time in the World, Princess Luna <=> Alcest - Les Voyages De L'Âme (Level 4)

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Artist Count:

=> Alcest - 4 songs

=> Alex M.O.R.P.H - 1 song

=> Angel Vivaldi - 1 song

=> BEING - 2 songs

=> Blood Stain Child - 1 song

=> Dark Tranquillity - 1 song

=> Disarmonia Mundi - 1 song

=> Do As Infinity - 1 song

=> Eluveitie - 1 song

=> Equilibrium - 3 songs

=> Eternal Tears of Sorrow (EToS) - 1 song

=> Insomnium - 4 songs

=> Kamelot - 2 songs

=> Lost Witness - 1 song

=> Maria Nayler - 1 song

=> Meshuggah - 2 songs

=> Mors Principium Est - 1 song

=> MyGrain - 2 songs

=> Nell - 1 song

=> Ne Obliviscaris - 3 songs

=> Nightwish - 2 songs

=> Noumena - 1 song

=> Oceanlab (Above & Beyond ft. Justine Suissa) - 6 songs

=> Omnium Gatherum - 1 song

=> Periphery - 2 songs

=> Peter Gabriel - 1 song

=> Piotrek Gruszka (Gru) - 1 song

=> Rush - 2 songs

=> Scar Symmetry - 5 songs

=> Solution .45 - 1 song

=> Star Breaker - 1 song

=> Widek - 1 song

Genre Count:

=> Djent - 2 songs

=> Folk Metal - 1 song

=> Industrial Metal - 2 songs

=> Melodic Death Metal - 20 songs

=> Melodic Power Metal - 4 song

=> Meshuggah - 2 songs

=> Neo-classical Metal - 1 song

=> Progressive Metal - 6 songs

=> Progressive Black Metal - 3 songs

=> Shoegazing - 4 songs

=> Viking Black Metal - 3 songs

=> Rock, Hard - 1 song

=> Rock, Soft - 3 songs

=> Trance - 10 songs

Hardness Level Count:

=> Non-metal - 8 songs

=> Level 3 - 2 songs

=> Level 4 - 3 songs

=> Level 6 - 5 songs

=> Level 7 - 10 songs

=> Level 8 - 21 songs

=> Level 9 - 4 songs

=> Level 10 - 3 songs