Bladetongue

Every hit sticks the target with ichor, which strips defense. Against anything heavily armored that is worth more than the sword's own damage, and it helps every other weapon you have on the bar for as long as it lasts. Some players keep it in a slot purely to open a fight, tag the boss, and swap to their real weapon.

Most projectile swords throw something on every swing whether or not you connect. This one only produces the spray when the blade actually lands, so swinging at empty air to fill a corridor does nothing. The upside is that catching several enemies in one arc fires one stream per enemy, so packed groups produce a mess of extra projectiles all at once.

Streams leave at random angles in front of you, drop under gravity, and bounce off surfaces a few times before fading. In an enclosed room that scatter works in your favor since everything ricochets back into the fight. In open sky a good share of it lands in the dirt and is wasted.

Fishing in the Crimson, at any depth, and it is among the least likely things in that pool. If your world is Corrupt you will never see it. The equivalents for the other biomes fill the same slot, so there is no reason to travel for this specific one beyond wanting ichor on tap.