Fire Feather

The feather comes off one enemy, and that enemy only starts appearing after you have put down a mechanical boss, so this is a mid Hardmode errand rather than something you trip over. The underworld is a poor place to grind casually. Red devils hit hard, they arrive mixed in with everything else that lives down there, and the broken terrain works against you. Flatten a platform arena, drop campfires and a honey pool on it, and the trip gets much shorter.

Its only purpose is the flame wings. If you already have wings from another source, do not sell the feather loose. Build the wings first and sell those instead, since the finished pair is worth far more than the parts that went into it.