Antlion Eggs

These sit in the background all through the underground desert and hatch the instant anything touches them, including a stray projectile or a swing you aimed at something else entirely. Early on that is how the desert kills you. One careless attack releases a cluster of larvae, they spit at you from off screen, and suddenly you are fighting six things instead of one.

Move slowly and stop firing at nothing. Once you can comfortably kill larvae, the better habit is popping every cluster you pass on purpose during a quiet moment, so none of them can go off behind you in the middle of a real fight. They also regrow on sandstone below the surface layer over time, so a corridor you cleared once does not stay clear. They will not appear beside another cluster, in liquid, or in front of walls suitable for housing, which gives you a way to make a route permanently safe.

Since you can place them yourself, they work as a spawner. Set a cluster down in underground corruption, crimson or hallow and the larvae that hatch count as enemies of that biome, so they roll for Souls of Night or Light and for Biome Keys. It is a slow key farm but it needs almost no construction.