You can find the original song that was remixed here:
https://www.youtube.com/watch?v=qrBB3_rFPjg

I stole the video from this:
https://www.youtube.com/watch?v=psplbU7MolE
Who stole it from this:
https://www.youtube.com/watch?v=TyNJt4e-3AA

I made this song, and you are free to use it for whatever purpose, although I would appreciate if you told me. But be aware this is a remix, so its not 100% mine.

You can download the mp3 here:
http://www.mediafire.com/download/ccodtvkgbuj8eff/Undertale+-+Spear+of+Justice+%28Ponyrumpus+Remix%29.mp3