Lord Williamson was a member of the Shadow Cabinet in life, and a ghost who haunted Chanceford after his death. He reluctantly told the Shades everything he knew about the Oswulf Stone before he was terminated by Rory, at his request.
Lord Williamson was an old, white-haired man in a gray suit from another century. His outline was firm, but faided.
Lord Williamson may have been born with the Sight, and spent his life trying to understand it. He recorded what he found out, but seemingly lived to regret it. He was a member of the Shadow Cabinet, and his one notable act in life was his decision to carefully move the Oswulf Stone from Tyburn to Marble Arch. This supposedly condemned him to stay on Earth as a ghost to watch over the stones near the house he built for that purpose.
Sometime before "The Shadow Cabinet" he talked to Stephen about the stones.
He agreed to tell the Shades what he knew if Rory would terminate him, which she did, although it was difficult for her to do.