A Witness is a mortal immune to the Sybaris of the Arisen. It's unclear why this would be but theories include Witnesses having souls fit for the Rite of Return, being the reicarnated souls of loved ones from ancient Irem, and being somehow in tune with the Fate of the cosmos. Some affinities and utterances can transform mortals into Witnesses, which further complicates the issue.
Regardless of their origin, being immune to the effects of Sybaris is a very useful skill for mortals to have and Witnesses themselves have an almost pathologic fascination with the Deathless. They make excellent mentors and retainers during a mummy's Descent and can rise high through the ranks of their cult.