Halo 3 ODST: Original Soundtrack - 04 Rain (Deference for Darkness)
This one is notable for being from a game I've never actually played. I half-watched Cross play this game, and this particular piece was just so goddamn emotional and perfect, and... Needless to say, I had to find it. It's so damn good. The tone it sets is impeccable.Bloodborne Soundtrack OST - Ludwig, The Accursed & Holy Blade (The Old Hunters)
My favorite game of all time, Bloodborne, delivers this piece alongside one of the best characters in the game. It starts off as dark and twisted, just another monster theme, albeit an expertly done one, bombastic and oppressive. But then, as it reaches the second half, the tone changes completely, and the music somehow becomes both dark and triumphant at the same time, doing a fantastic work of illustrating what a twisted monster Ludwig has become - but also that the man he once was is still in there, somehow making it all so much worse.Dark Souls OST - Nameless Song
For the Dark Souls series, rather than a grand battle theme, I chose this little ditty. It's much like Rain, a mood piece. Not much to say about it - it's somber, but also with a glimmer of hope, just like the world the game takes place in.Dark Souls III Soundtrack OST - Abyss Watchers
But, if you want a battle theme, there's this. The Abyss Watchers theme is incredibly sad. You get the feeling that a great era is ending as the song plays, and that's pretty much what happens during this fight, and indeed, all of Dark Souls 3. Abyss Watchers is, musically, the gold standard for me as far as Dark Souls boss music goes, though Gwyn, Lord of Cinder is a close second.Nioh OST, Yuki-Onna / Tachibana (Boss theme)
Meanwhile, we have Nioh. Not taking place in a dying fantasy world, but rather a fantasy version of feudal Japan. And in that world, nothing gets me more pumped than this song. It's such a fucking excellent battle theme that really takes you into the battle and makes you feel like a samurai.Pokemon Sun & Moon - World Champion Battle Music (HQ)
A modern take on a classic battle theme - namely, Blue's from the original Pokemon Red and Blue (Or Green, depending). It's a great theme, and this rendition definitely does it justice. An epic song for an epic battle.Fallout New Vegas Music-Ending SlideShow Theme (Extended)
My second favorite game of all time, Fallout New Vegas, plays this music at the end of both the game itself and any expansions. It's both uplifting and sad at the same time, showing what you've accomplished on your journey, but also marking its end. War never changes. But people do, through the roads they travel. And that's what this piece of music is all about.Undertale - Megalovania
You feel like you're gonna have a bad time.
That's all I got just off the top of my head.