Also note: The original Underfell creator re-opened their blog and have been posting once in a a while over the past several months/a year/what is time even?
They've added more headcanon material (though they said you're, of course, free to use your own interpretations), lots of goofy pictures (and some serious ones), updated character details, and mini-comics, etc. :3
I'm so glad they're back!