Hellfire Arrow

It detonates where it lands and hits everything in a small radius, which is the pre-Hardmode answer to enemies that arrive in clumps. Bone Serpents and other segmented enemies take several hits from one arrow because the blast catches neighboring segments. Despite the name it sets nothing on fire and applies no debuff at all.

Use it with a bow that fires one arrow at a time. The explosion applies invincibility frames that the arrows themselves respect, so a multishot bow throws most of its shot away: the extras phase harmlessly through a target that is briefly untouchable. The interaction is easy to miss, because the arrows look like they are connecting.

The Underworld hands them out through its chests and pots, and gold chests that generated deep enough sometimes carry them as well. Crafting is easy once hellstone is flowing. After the Wall of Flesh the Demolitionist stocks them, which is usually where you stop thinking about supply. One last thing: dropping one into shimmer converts it into a Shimmer Arrow instead of breaking it back into parts, which quietly closes an early shortcut to hellstone bars.