Undertale

Toby Fox wrote, composed and largely programmed this himself, and it became one of the most discussed games of the 2010s on the strength of a single design decision: every enemy can be spared instead of killed. That option is not a morality meter bolted onto a combat system — it restructures everything. Fights are bullet-hell dodging sections where the way out is usually social rather than violent, the cast remembers what you did, and the game tracks choices across playthroughs in ways it is better not to spoil. The 2015 release is short, deliberately cheap-looking, and much funnier than its reputation suggests, which is part of why its sharper turns land. The soundtrack is the other half of its success, doing character work through leitmotif at a level rarely attempted at this scale. Its two headline routes pull in opposite directions: one is among the warmest things in the medium, the other is openly hostile to the person playing it. Fox's follow-up Deltarune has been arriving in chapters ever since.