As most of you should know, yesterday, a 3-hour orchestral Undertale concert premiered on the official Undertale YouTube channel. It was a great performance with some amazing renditions of Toby Fox's work. At the end, however, is a ten-minute (pre-recorded) piano performance of Toby Fox playing some arrangements of his own music. On Twitter, he wrote:

"Thanks for listening to my messy piano playing at the end too. The first piece was an arrangement of a song from DELTARUNE, the last two were UNDERTALE songs with improvisations. The middle one especially (Snowdin arrangement) I just randomly made up for the concert. Also I wasn't playing live, the whole video was pre-made. Also don't worry about my wrist, I only did a couple takes then edited out some of my worse mistakes. Of course, I made so many mistakes, I couldn't remove them all. LOL."

Here is a link to the full Undertale concert:    • UNDERTALE 5th Anniversary Concert  .
The music starts at around 50 minutes in, but there're some cool Easter Eggs beforehand.