Luna's Runic Pixie Hood

Another developer outfit tied to Treasure Bags, which means it cannot show up in a Classic world at all. Every Hardmode boss bag can roll it except Queen Slime, and the chance is low enough that it normally arrives while you are farming a boss for something else entirely.

It is named for Luna, an artist at Re-Logic who works under the name Runic Pixels.

The clothing is social slot decoration and does nothing for your survival, but Luna's Runic Pixie Wings drop in the same package and those are functional flight rather than a costume prop.

If you are chasing this, chase it for the wings and treat the outfit as the bonus. A second copy of the clothes is only worth keeping if you want one version dyed differently.