i.e. what songs do you consider to be 'themes' for the villains and protagonists of the show?

Nightmare Moon (Nightwish: Scaretale)

Blueblood {if he were to get in control of Canterlot} (Megadeth: Symphony of Destruction)

Sombra (Sabaton: Carolus Rex)

can't think of songs to suit the others off the top of my head

3798795 I really only have one, and it's for Nightmare Moon. Definitely one of my favorite songs by one of my favorite bands.

Nightmare Moon:

3798795

Obligatory:

This one kinda reminds me of the "Hearth's Warming Eve" story and the wendigos, particularly the line "a nation born of hate" (since the leaders of the three tribes started off hating each other):

This kinda gives me the feel of a battle between Celestia and Nightmare Moon (especially with the part around 1:50)

I'll try to post more if I can think of more...

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Nightmare Moon

King Sombra

Chrysalis

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I have absolutely no idea why, but I could see Nightmare Moon with this theme song. One of my favorites, by the way.
https://m.youtube.com/watch?v=6DUCqBkuu7E

Sombra:

I just don't know why it fits for him from my PoV.

Discord, for certain... XD
https://www.youtube.com/watch?v=majaL5_qFu8

And for some reason, Tirek always makes me think of Shinigami...
https://www.youtube.com/watch?v=fenwJjXbhUg

And the changelings...
https://www.youtube.com/watch?v=6mPQVvd28WI

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How's this one for Discord?

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Discord:

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Nightmare Moon - Lady Evil (by Black Sabbath)

Queen Chrysalis - Alucard (by Satan's Satyrs)

King Sombra - Kill the King (by Rainbow)

(Oh, it's kinda hard to make out the lyrics on the Satan's Satyrs song, so here's the lyrics:
Out of the coffin, down on the scene
My task: to infiltrate
Ritual wear is the bellbottom's flare
In this age of haze
I'll don it!

Archetype of evil dispatched on city streets
Creep in the cathedral, in a casket sleep
Does the fire in my eyes betray my groovy guise?
Stare a little deeper, you're hypnotized

That's right!

Seduce the birds with hollow words
"Baby, Satan's chic"
Back to the pad, show 'em where it's at
Bleed 'em at the Master's feet

You know I will!

Archetype of evil dispatched on city streets
Creep in the cathedral, in a casket sleep
Does the fire in my eyes betray my groovy guise?
Stare a little deeper, you're hypnotized)

Nightmare Moon: Mayhem - Freezing Moon

King Sombra: Burzum - A Lost Forgotten Sad Spirit

Tirek: Napalm Death - Scum

Queen Chrysalis: Tool - Stinkfist

Tirek

I could totally see him as a nazi if he were a person

Discord: Eeyore - Slipknot

Nightmare moon: Lord Nefaryus - Dream Theater

Chrysalis: The Beautiful People & Angel With the Scabbed Wings - Marilyn Manson

Teirek: Monstrosity - Meshuggah

Sombra: Walk With Me In Hell - Lamb of god