Who they were
As Grand Historian of the Han dynasty, he continued the historical project begun by his father Sima Tan.
What they did
In 99 BC he defended the surrendered general Li Ling, angering Emperor Wu, and chose castration over execution so that he could finish his history. The result, the Shiji (Records of the Grand Historian), covers everything from the legendary Yellow Emperor to Emperor Wu in 130 chapters, and created the annals-biography format that every later official dynastic history followed.
Legacy
He is remembered as the founder of China’s historiographical tradition. His letter explaining his choice to endure punishment and complete the work is a classic of Chinese prose.