Rain Song

It is an instrument, not a weapon. Which chord comes out depends on how far the cursor sits from your character, so you play it by sliding the mouse in and out rather than by pressing anything different. Six chords laid out from close to far, and the game prints the current one under the cursor while the guitar is held, which is what makes it possible to play something deliberately instead of mashing.

Getting one is the tedious part. Flying Fish only appear while it is raining, and the drop is rare, so you want a storm, an open stretch of surface where they swarm, and a weapon with wide coverage to clear them in bulk. It is worth setting up during any rain you happen to be caught in rather than waiting for one.

RainSong is a real maker of carbon fiber guitars, and the internal name for this item calls it a carbon guitar outright. Redigit has said the design in the game is based on the one he owns.