Tiara

This is the stock most people never notice, because the Clothier only offers it in Hardmode and only on a new moon. Phases advance one per night, so if you check his list and the pieces are missing you have several in game days to wait before the window comes back. Buy the tiara and the dress on the same night while both are there.

The dress is drawn after Princess Zelda in Ocarina of Time. The tiara copies something else entirely, the headpiece Cecil wears in his paladin form in the 16-bit Final Fantasy IV, changed only by a sapphire sitting where the ruby was. Wear it with Hallowed Plate Mail and Hallowed Greaves and the silhouette lines up with that paladin armor almost exactly, which is the combination the set is really there for.

A second Princess Dress exists in the game, the one that comes out of Goodie Bags, and it looks nothing like this one. Only two other item pairs share a name that way, so when you are digging through storage for the piece you paid for, go by the sprite rather than the label.