FTP, name this Persian emperor who was succeeded by his son Xerxes I.
He ascended to the throne by defeating a usurper who pretended to be this man's murdered uncle.  In one of his notable administrative acts, he reorganized his empire into 20 sections.  Near the end of his life, he ordered a large limestone rock to be inscribed with his genealogy in three languages; that inscription was translated into English in 1849.  He also led an expedition against the tribes of the Danube River in 516 BC, put down a rebellion of Ionians in 499 BC, and attempted to subjugate Greece with his son-in-law Mardonius, but his forces were defeated at Mt. Athos and Marathon. FTP, name this Persian emperor who was succeeded by his son Xerxes I.
He ascended to the throne by defeating a usurper who pretended to be this man's murdered uncle.  
Darius I (accept Darius the Great; prompt on partial answer)