The costume is meant to be the one-year anniversary gift to celebrate a whole year since the release of Xenoblade Chronicles 2. It is the only Aux Core-based Costume that is available both in Xenoblade Chronicles 2 and Torna ~ The Golden Country.
The costume itself gives Mythra black spandex that covers her legs and back, giving the appearance of a spandex suit under her standard white dress. The open gap in the blouse of her dress is also covered up. The design itself is based directly off the altered artwork of her Spirit that appears in Super Smash Bros. Ultimate.
The Aux Core can be obtained from Brobro on the Salvage Deck of the Argentum Trade Guild after completing Chapter 4 in Xenoblade Chronicles 2, and from the Nameless Wanderpon in Feltley Village in Torna ~ The Golden Country.