Wang Xizhi ( 303–361) ) is one of the most famous calligraphers in Chinese history. He lived in the Jin Dynasty (265-420). He was regarded as the sage of calligraphy. His calligraphy, especially his running script calligraphy, was most precious for anyone who is interested in Chinese calligraphy. Tang Taizong, one of the emperors of Tang Dynasty, loved his calligraphy so much that he ordered lots of copies of his work. It's also said that Wang Xizhi's most famous work, the Preface to the Poems Composed at the Orchid Pavilion was buried with the emperor in his mausoleum. So nowadays people can only enjoy the copies of the original one.
The following video from ChinaX will give you more detail of the legend of Wang Xizhi and his well-known Preface to the Poems Composed at the Orchid Pavilion.