Undertale Yellow is a fangame and "prequel" to Undertale. If you liked the original game, chances are high that you'll like UY. It's free and you can get it on the link below

https://gamejolt.com/games/UndertaleYellow/136925

The game does a great job in keeping its universe believably canonical by sidestepping most characters from the original game. You interact with Toriel for a very brief moment in the starting 2 minutes, then you're out to explore another side of the underground. Only Flowey will follow you around and, if you've played the original game and understands how and why he did the things he did, everything he does in UY will make sense. The game doesn't spoil any of the revelations that happen in the OG and only mentions the important characters (Asgore, Toriel, etc) sparingly and without extrapolations, so it really does a good job of acting as a prequel that respects the canon and doesn't extrapolate.

The new characters really feel like they belong in the OG, with a neutral/pacifist run showing how silly, dorky and lovable they are, this is true even for the background NPCs and enemies you fight. Not every such NPC is lovable, some are kinda assholes, and that's a good contrast.

Perhaps most important, the soundtrack is fucking awesome (the Gamejolt link above also lists the soundtrack for listening. 77 - Meltdown, for one of the later areas, rocks). Some are references and/or homages to original tunes. Boss themes are super memorable and really fit the respective characters.

One thing that I loved is that every enemy has a "hurt" graphic, which shows once they're under 1/3 of health, which also lets you spare them. Seeing them hurt like that does make me feel kinda bad.

Oh, a genocide run won't be a cakewalk like the OG, you'll be up against a number of alternative bosses that will give your reflexes a workout (I've died on the first genocide-exclusive boss 8 times, and that's after a 0-death, up to the final boss, neutral run).