Lady Scintilla, known as the "greatest of the Visionaries", was a prisoner on Shada. She was imprisoned for "conspiring with Carrionites".
Scintilla became a figure of Gallifreyan legend, appearing in books such as the children's history Our Planet Story. In that book, she was depicted as "a haughty flame-haired woman in red robes", with "six-inch razor-sharp nails painted blood red". Scintilla did have long, sharp nails, but she was actually short and dumpy. The book was read to the Doctor as a child and was one of the primary reasons for him wanting to explore time and space.
Scintilla was a "natural born" Time Lord, meaning that it was both harder to kill her and harder to cause her to regenerate. (PROSE: Shada)