Undertale

Toby Fox made Undertale largely alone using GameMaker Studio, funded by a $51,000 Kickstarter. The game lets you complete every encounter without killing anyone — a radical concept in 2015 RPGs. The genocide route, where you kill everything, punishes you for it permanently by altering future playthroughs even after deletion. This meta-awareness, combined with Fox's brilliant writing and a soundtrack that spawned "Megalovania" (2.6 billion Spotify streams), made Undertale a cultural phenomenon. Over 10 million copies sold. Fox then made Deltarune. He's still largely working alone.