Tiresias is a character in the novelization of Time of Zygon. He is described as a 'colleague' of the Doctor and a soothsayer (not to be confused with The Soothsayer)
He plays an important role in the progression of the story. At an undisclosed point in the Doctor's fourth incarnation, he travels back to Gallifrey to acquire the Swords of Rassilon. Tiresias foresees a point in the future in which the Doctor will use them, and thus release forces of "silence and ending" that will bring "darkness to time".