The paladins of the Sapphire Guard are aware of the existence of the Crimson Mantle, and under the direction of Soon Kim sent out crusades to eliminate it. The mantle survived these efforts, albeit a little tattered, and was passed to a new cleric each time the previous bearer was killed. In this manner it was passed down to Redcloak's Revered Master, and then to Redcloak. Redcloak has worn the mantle for 34 years.
The Crimson Mantle slows the aging process of its wearer, allowing one goblin to accomplish more over the course of several lifespans. It also protects the wearer against disease, including magical ones such as Lirian's Guardian Virus. The wearer of the Crimson Mantle further gains all of the Dark One's knowledge regarding the Gates, and his plan to manipulate them.