Complex sentence: The text was written in Classical Chinese (as used in writing by literate Koreans at that time) by the Buddhist monk Ilyon (1206-1289) at the end of the 13th century, a century after the Samguk Sagi.
Split into simpler sentences: The text was written in Classical Chinese (as used in writing by literate Koreans at that time). It was compiled, at least in part, by the Buddhist monk Illyeon (1206-1289) at the end of the 13th century, a century after the Samguk Sagi.