The Pageant was a group of other-dimensional beings who wore masks to give themselves identity and could alter reality through their machinery. Their culture was a surreal, haunting blend of science, art, and theatre.
Curious about Earth history and philosophy, they took it out of the main universe around 1794. They made alterations to it and placed it in another dimension to see what would happen. The project's leader was Masksmaster Robin Goodfellow, who oversaw affairs from inside the World Machine. They did not intend to cause harm to the human race, but the experiment got out of hand when Minski went rogue. Once the Pageant resumed control, they restored the Earth to its original location in space and time and promised not to perform any similar experiments.
The Pageant hinted at intimate knowledge of the Time Lords of Gallifrey, and they referred to the First Doctor as one would refer to aristocracy, addressing him as "my lord." The Doctor was notably displeased being called this. (PROSE: The Man in the Velvet Mask)
Along with Faction Paradox and the Ministers of Grace, Masksmakers of the Pageant went back in time to help the effort against the Mad Mind of Bophemeral in the Millennium War. (PROSE: The Quantum Archangel)
Behind the scenes
Due to Daniel O'Mahony's focus on ambiguity and surrealism in The Man in the Velvet Mask, the full scale and finer details of the Pageant's culture is not clear. The full responsibilities or significance of the role of Masksmaster, other than leading the Pageant's experiments, are unknown. The novel also makes a brief mention of "the Graces," but it is unclear who or what these are, if they are even part of the Pageant at all. Craig Hinton's use of the term "Maskmaker" is limited to his novel The Quantum Archangel.