The most frequently used name for Al-Mahdi's (as) mother is Nardjis. She was from the Roman Empire, the most powerful European state of that time. Muslims defeated her father and she was taken as a prisoner of war. The Imam of that time was Imam Hassan Al-'Askari (as). Nardjis had seen Fatima (as) in a dream and embraced Islam, and later she married Imam Hassan Al-'Askari (as) and gave birth to Al-Mahdi (as).33
33. Araki, Mohsin, Ideological and Jurisprudential Frontiers of Islam, p.26, Book Extra, London, 1999
Adapted from the book: "The Awaited Saviour; Questions and Answers"