This infection turned them mad and made them attack other non-infected bugs. Also all of the citizens are insects or insect-basesd creatures. The goal of the game is to defeat the Hollow Knight who’s hosting a malicious entity called the Radiance that’s infecting the denizens of Hallownest with a disease that turns them mad. As you travel you unlock upgrades that give you abilities to reach other parts of the map, you can find charms and spells that give you fighting abilities, and the core components are parkour exploration and fighting enemies. You play as The Knight, nicknamed Ghost, who enters the kingdom from the outside and traverses it. For a little bit of backstory the game takes place in Hallownest, a shell of a former kingdom left to ruin. Item descriptions and NPC interactions help give context, and when one begins analyzing the map and characters met along the way the pieces of the puzzle slowly slot into place. There’s a lot the player does to complete the game, but the why is often unspoken. The game very much leaves the worldbuilding up to the player.
