Here is the source to Undertale. I have no idea how long it will stay availible. The password to the archive is dapple because it wouldn't let me upload otherwise, I think it was the music. Anyhow, this game was created in Game Maker, so it wasn't rocket science.
This is a "decompiled" source, and will not easily work compiling it back with GM. I am working on that now, but it may be some time. You have to manually re-introduce each asset and fix broken linking.
I hope to have a compiling version sometime soon, but no promises. If someone else gets the initiative, then have at it.
Also note that Game Maker assets are compiled, which means you will either need to re-introduce them into Game Maker to export/use them, or you will need to use the Game Maker api and decompile them that way.
I am curious to see if this helps with finding any further secrets. It could also help you learn a bit about game development. I am just to busy for sifting through it.
https://www.dropbox.com/s/u3vj5a3modwbndi/Undertale%20SRC%20TheDruid.7z?dl=0
Again, password to archive is:
dapple