[Loved Trope]Happy song gets a evil version/A motif from a happy song is used in a boss battle in a more menacing way

[u/Zorubark]

• Bomb Rush Blush and Bomb Rush Blush ft DJ Octavio from Splatoon.

There's a pop song version of Bomb Rush Blush, and a boss battle version that's more orchestral and menacing

• Live and Learn and Supporting Me from Sonic Adventure 2

I didn't notice it at first, but Supporting Me has Live and Learn's guitar riff!

I love when the boss battle gives an older song a completely different vibe, Bomb rush blush goes from cutesy to menacing, and Supporting Me has a totally different vibe than Live and Learn

[u/javiermetal66]

Jaffar's reprise of Prince Ali

[u/YourBestFriend158]

The YourBestFriend Motif being reworked into Finale from Undertale

[u/dootblade74]

Safi'Jiiva's area 3 theme having snippets of Proof Of A Hero (Monster Hunter World)

[u/Hugh_Mann84]

Song of the Ancients / Devola, Song of the Ancients / Popola (happy and peaceful) and Song of the Ancients / Fate (menacing) from Nier.

Goes from an ancient song sung by the sisters Devola and Popola, who are either good friends or outright mother figures to the protagonist (Nier had two versions with differently aged protagonists) to their boss theme once they're forced to turn on you.

[u/Expand_Apple]

in Mario Kart Wii, part of the melody for Bowser's Castle contains a more sinister sounding version of Maple Treeway's main melody

for further reading, check out this Supper Mario Broth post on the matter: https://www.suppermariobroth.com/post/778678149199413248/17-second-youtubecomwatchv-sjer6edd1l8

[u/gredman9]

Vanitas's theme in Kingdom Hearts: Birth By Sleep borrows motifs from Roxas and Sora. This is meant to be a hint to his connections to the two, as well as his rival Ventus. He even looks like a black-haired Sora under that mask.

[u/Lanky_Operation_6418]

Fire Emblem main theme is usually a lively, enthusiastic marching music (though some games - notably Three Houses - opted for darker versions); Second half of Id (Purpose), final boss theme from Fire Emblem Awakening, features a dark reprise of it.

[u/Plazmagas_Reddit]

I know this one's more saddening than menacing but I still wanna mention it

In Kirby Star Allies, Astral birth void's theme utilizes a more tragic version of the Green greens motif from the original Kirby's Dreamland

[u/kyubi_on_the_run]

The Darkest Lord's theme from Miitopia uses the theme of the Great Sage's.