The piano piece featured in Undertale: Piano Choir is a piece of music that was created by Undertale composer, Shigeru Miyamoto, in the game’s final episode. 

In the song, the player must make an arrangement of strings and piano keys in order to defeat the mysterious Undertale.

Miyamoto used an array of different instruments to achieve the song’s musical arrangement.

Undertale composer Shigera Miyamoto and Undertale musician Shigerun Miyamoto. 

The Undertale Piano Choirs is a game-exclusive song.

It is available to download for free through the game itself and to purchase in-game for 5,000 yen.

You can find the full score of the song on YouTube. 

There are no songs to download, so if you have not tried the song yet, you might want to give it a listen.