This can be for any au you want (Napstablook as Undyne only), provided you give credit to original creator of the song. Please ask beforehand.
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Request list (Role-swap themes):
1. Chara!Napstablook - Ghost Fight
2. Papyrus!Asgore - ASGORE
3. Asriel!Mettaton - Metal Crusher
4. Fear the Undead V2
5. Corrupted Justice - Heartache Replacement (Alphys)
6. Alphys!Asgore - ASGORE
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Leitmotives (In-order):
-Ghost Fight
-Spooktune
-Chill (Subtle)
-Thundersnail

Source of Image: https://appulsprite.tumblr.com/image/141317041638

Youtube: https://youtu.be/eSLUac5O6cY

[Swapspin/Underscramble/A Napstablook Battle Against a True Hero] Last Tune in the Album

You still have no idea who I am... Ok some things about me, just a really busy 16 year-old guy who's spontaneously producing au tracks. I take requests ocassionally or when i have more free time. i work on many aus (including someone's au) and several UT fangames. I make roleswap au tracks a lot more than crossover ones. I mostly make death by glamours, battle against a true heroes, heartaches, and asgores because those are my favorites and really fun to make. I don't make megalos (really sorry but I'm just not really into that) though I make custom genocide run final bosses at some point.

And here's my discord: MoonStriker#0730

QnA:

Q: Do you have any AU?
A: Yeah, I have: Rolled Cycle (Swapspin), Last Hope, and Underswitched. "Will I make songs for one of theses aus?" Someday, yeah. 
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Q: Can I use your musics for my au?
A: Sure! If the description of the certain music says that, then yes! I'm wary about Deltarune aus, though.
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Q: Do you have any song project?
A: I consider "Chara and Asriel swap" to be my primary music project. There's more such as "Chaos Kings" and "Doused Flames". Upcoming project might be "Toriel and Asriel Swap" or "Mettaton and Asgore Swap". I'm not sure, but you can decide which one too based on what you're interested with.
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Q: Do you take request?
A: Yes, but that depends or it'll take some time to actually do it.
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Q: What is your favorite UT music?
A: For battle theme: Battle against a True Hero goes to number one! Death by Glamour goes to second! And finally... ASGORE goes to third! Last but not least... Heartache or Spear of Justice? I don't know, both are on-par to me =P As for area theme, Waterfall and Another Medium for the win!

[Swapspin/Underscramble/A Napstablook Battle Against a True Hero] Last Tune in the Album

MoonStriker (Archive II)