Darius the Mede is a biblical person in the Book of Daniel, Chapters 6-11, who rules over "the kingdom of the Chaldeans" after the last king of the Babylonian Empire (Belshazzar) is deposed by the Medo-Persian armies. The author of Daniel indicated that Darius was about 62 years old when he "was made king over the kingdom of the Chaldeans". He is best known for having been forced into throwing Daniel in the lions' den.
No such person as Darius the Mede is known in history. There have been attempts to identify Darius the Mede with such persons as Cyrus the Great, Darius the Great or Gobryas.
Read more about Darius The Mede: Historicity, Identifying Darius The Mede, Jewish Views