Mist Cloak is one of the leaked items from either Floor 9, or the 2016 Event floor. Multiple players have been getting this armor from admins.
The Mist Cloak closely resembles the Coat of Midnight that Kirito wears in the anime Sword Art Online. Except in the anime Kirito wears his cloak over the Fields Light Armor instead of the Blue Starter Clothes.It is a black coat with white shoulder pads, over Blue Starter Clothes.
Many players have already duplicated this armor which isn't technically what you can call rare nowadays.However this armor has the same defense as the Rogue's Plate making it very useful in battles.This is practically useless for duels for it's speed.