Perchance to Dream (Side Quest)
After completing the Master of Secret Things quest in the real world, and each of the four afterlife side quests (Love or Duty in Aaru, Blade or Shield in the Duat, Gods or Creed in Aten, and Follower or Leader in Heb Sed), return to the dream master for a chat.
He'll interpret your afterlife adventures as dreams (albeit lethal dreams where you can collect loot, speak to the recently departed, and kill spirits that are affecting the real world in material ways). Completing this conversation is all that is required to complete the mission.