The Chronicler

The Chronicler was a Time Lord who watched reality through a holosphere and recorded what he saw through quill on parchment. He worked in a great hallway lined with stained glass, but was sometimes so engrossed in his holosphere that he was oblivious to it.

When observing the Fifth Doctor defeat Nephthys, the Chronicler felt that the Doctor had almost cheated by forcing her to travel through time until she aged to death. The Chronicler used the holosphere to change reality so that Nephthys could not be defeated that way. When the Doctor adapted and reacted to the different situation, the Chronicler returned reality to its previous settings and took a break from his work. (PROSE: The Sands of Time)

Behind the scenes
The Chronicler does not appear in the original version of The Sands of Time, instead only being introduced in the 2004 e-book edition, as a way to insert an alternate ending into the story. The character bears a similarity to the Librarian, another Time Lord-related archivist of reality who encounters the Fifth Doctor in Catalogue of Events.