Samguk Yusa is a collection of legends, folktales, and historical accounts relating to the Three Kingdoms of Korea (Goguryeo, Baekje and Silla), as well as to other periods and states before, during, and after the Three Kingdoms period.
The text was written in Classical Chinese, which was used by literate Koreans at that time. It was compiled, at least in part, by the Buddhist monk Iryeon (1206 - 1289) at the end of the 13th century, a century after the Samguk Sagi.
Unlike the more factually-oriented Samguk Sagi, the Samguk Yusa focuses on various folktales, legends, and biographies from early Korean history. Many of the founding legends of the various kingdoms in Korean history are recorded in the book. It includes Gojoseon, Wiman Joseon, Buyeo, Goguryeo, Baekje, Silla, and Gaya.
This is the earliest extant record of the Dangun legend, which records the founding of Gojoseon as the first Korean nation.