Fox's most well-known work is the 2015 role-playing video game Undertale. The game sold more than 1 million copies,[6] becoming a "breakout hit" and "pop-culture phenomenon".[7][8][9][10][11][12][13][14] Fox worked on the entire game independently, besides the art assets—which he asked Temmie (Tuyoki) Chang to help with—to avoid relying on others.[15] He had some experience in game development before Undertale, using RPG Maker 2000 with his three brothers to make role-playing games and EarthBound ROM hacks in high school,[16][17] the most notable of which was Earthbound: The Halloween Hack.[18] He thought of the character designs and ideas for Undertale while in college, where he drew them in his notebook.[19]