I was doing the Portico in Celestia and when I fought the 3 Henchmen guys, they all appeared very transparent, almost invisible. For specifics, it was Thrale Bonestriker, Servus BloodSword, Vagen Soulbreaker, and all of the Shadow-Web Conscripts they had. I'm not sure if the same goes for the Shadow-Web Mercenaries, Celestian Revanents, or any other faded Mob/Boss out there, but I would suggest to look into this.
