Let me say right here: I LOVE THIS
I love all your leads and chords and pads, EVERYTHING!
Really nice diversity and mixing, as well.
One note about the musical aspect of it, however, there are some notes in the melody at the drop (about 2:39 ish) that are way out of place that make it very distracting. Now I'm not very familiar with the music from Undertale, but my guess is that you used a melody and put chords behind it. That can be risky, because if you don't have perfect pitch or something, it easy to not recreate chords from a video game to a DAW, but you gotta make sure that you have it spot on, or else it will sound weird.
Overall, very nicely done. Pm me about any questions, because i kinda went all out on this one xD