Brown Driver Briggs

 Darius = "lord"
1. Darius the Mede, the son of Ahasuerus, king of the Chaldeans, who
succeeded to the Babylonian kingdom on the death of Belshazzar;
probably the same as "Astyages" the last king of the Medes (538 BC)
(same as 1867 (1.)
2. Darius, the son of Hystaspes, the founder of the Perso-Arian dynasty
(521 BC) (same as 1867 (2.)
3. Darius II or Darius III
  1) Darius II, Nothus (Ochus) was king of Persia during the time
of Nehemiah (424/3 - 405/4 BC). Darius II is the most probable
because he is mentioned by Nehemiah and ruled during the time
of Nehemiah
  2) Darius III, Codomannus was king of Persia during its last years
at the time of Alexander the Great and was his opponent
(336 - 330 BC)