Crowno's Mask

Crowno's outfit is a developer costume, and it only turns up inside Hardmode boss treasure bags, so an Expert or Master world is the entry fee. Queen Slime is the exception and her bag never carries one. Anything else in Hardmode that you can resummon on demand makes a fine farm, and the roll is rare enough that you should plan on opening a lot of bags. The wings drop alongside the clothing rather than on their own roll, so one lucky bag hands you everything at once.

The clothing is appearance only and belongs in the social slots. The wings are not: they fly like any other pair, which makes them worth holding onto even if the costume never goes on. The headpiece looks a lot like the mask the Lunatic Cultist wears, close enough that people mix them up at a distance.

Crowno is one of the artists who makes the game's graphics. The set existed in the files well before anyone could legitimately get one, and back then it fought you: wearing a piece dragged your movement speed down and stacked a pile of debuffs on you until you took it off. All of that came out when the set became a real drop.