Undertale is without a shadow of a doubt the single most important video game of all time for me, if not one of the single most important things in life for me. I was 10 years old when the game released, and was immediately hooked, and throughout these past 8 years, Undertale has slowly but surely become an absolutely massive part of me, it's one of the reasons I am who I am, and one of the reasons I'm still here.
This is also one of the reasons I'm very picky when it comes to fan-content.

So I hope you understand that when I say the final boss of this fangame inspires within me the selfsame feelings that Undertale's does, I mean that this is unambiguously the absolute pinnacle of Undertale fan-content.
More than anything else I've ever played/listened to/watched, Undertale Yellow really gets what makes the original so special in a way nothing else really does, and I'm gonna be honest, it still doesn't feel real. I finished the game just a couple days ago, and I still catch myself thinking stuff like "There's just no way Undertale Yellow actually happened, right? Like there is absolutely no way I actually just played Undertale Yellow, and that it was that good."

This game was in development for over 7 years, 7 entire years. I was 11 years old when the demo came out. It's such a surreal experience. How this game exists is beyond me, it's damn near a miracle, and I couldn't be happier.
Safe to say, I will be thinking about this game for a long, long, long time.

My only real criticism is... the entire no mercy route. I find it a bit odd that it not only exists, but is this extensive.