Purple Robes are the servants of the Lords of Judgment who reside in the Paths of the Dead and the Halls of Judgment.
Souls of individuals who fail to negotiate the Paths successfully or are deemed unworthy by the gods are typically recruited as Purple Robes.
Purple Robes apparently have little (if any) free will of their own, and are at the beck and call of whatever the gods, or other free souls occupying the paths, may wish.
The typical service of a Purple Robe will last for thousands of years, after which time the gods may allow the soul to reincarnate.