Unholy Arrow

These are the arrows you buy rather than farm. Once the world evil boss is down, the Arms Dealer stocks them after dark, and in hardmode he keeps them on the shelf during the day too, so ammo stops being something you plan around. The Eye of Cthulhu also hands over a starting supply, which is usually the first time a player sees one. Crafting works before either of those, but it is slow enough that most runs wait for the shop.

They punch through several bodies in a row and give up only a sliver of damage on each pass, a much gentler falloff than the other piercing arrows carry. They give off a faint glow while traveling, so a volley into a dark cave doubles as a look at what is waiting in it. Nothing is recovered once fired, unlike Wooden Arrows, so treat every shot as spent.

Enemies strung out in a line or scattered across the screen are the ideal target, and Old One's Army lanes are the clearest case for them. Against a tight cluster or a worm, Hellfire Arrows put out more, because an explosion that overlaps the same body repeatedly beats one clean pass through it. Hellfire is the only pre-hardmode arrow that hits harder and you cannot buy it, so plenty of runs end up carrying both and switching by target.

Jester's Arrows pierce more targets and travel through blocks, but they bleed damage twice as fast on each hit and they wink out on their own after a short flight. Unholy Arrows keep flying until they meet something. For the Eater of Worlds or the Brain of Cthulhu the Jester's still wins on sheer segment count, and Fallen Stars are less work than farming the evil biome for materials. Everywhere else, being able to walk into a shop and buy a stack settles the question.

The recipe changes with your world evil and it is one of the very few that is not balanced between the two. There is no Crimson counterpart to the Worm Tooth, and Vertebrae are easier to collect in the first place, so the Corruption version returns more arrows per material to even it out. Nothing you can do about it, but it is worth knowing before you assume a Crimson world is being short changed.