MarkusMasterThief
That would be AmethystDeceiver's Black Sacrament armor! Here's the download link: http://www.nexusmods.com/skyrim/mods/5696/?
It comes with a mini-quest where you have to track down and dig up the remains of five ancient assassins to get their armor.
It comes with a mini-quest where you have to track down and dig up the remains of five ancient assassins to get their armor.