Undertale Yellow is a fangame that serves as a prequel to Toby Fox's Undertale. It features a new child named Clover, branching paths where choices matter, new characters, three main routes, a Run button(!), and 135 new songs and remixes.
A free download, its difficulty level is slightly higher for those who have already completed Undertale. Still, it has accessibility options to make it easier for everyone to play. Naturally, it also contains spoilers for the original game.