Phantom Thieves "Dusk" (怪盗団“黄昏” Kaitō-dan Tasogare) is an organization in the Black World.
Z/X Encyclopedia Entry
In the early days of the Black World, thousand masks created with super technology produced thousand Diabolus, and created an environment where the concept of reckless death prevails. Masks that lost their owner after they killed each others in order to gain more lifespan sometimes meet their next owner, and sometimes abandoned entirely—. Many of them are gathered by collectors who took a liking to them.
Phantom Thieves "Dusk" are people who raided the residences of such collectors. Courteously sending calling cards to them beforehand.