Ichor Arrow

The point is the debuff, not the arrow. Ichor strips defense off whatever you hit, so everything else you throw at that target lands harder, including the rest of the arrows. Against a boss with heavy armor the swap is worth it even when a stronger arrow is sitting in the next slot.

Open with enough shots to stick the debuff, then switch to your damage arrow and refresh it whenever the tint fades. Against a swarm it is less useful, since spreading the debuff around costs you more shots than it saves. In player versus player the debuff lasts noticeably longer, which is why it shows up in duels more than the numbers suggest.

Crimson worlds get these. Corruption worlds get the Cursed Arrow instead, which burns rather than shreds armor. If you rolled the wrong evil, an artificial biome or a second world is the only way in.