Year 921 (CMXXI) was a common year starting on Monday (link will display the full calendar) of the Julian calendar.
The Fatimids crush the Idrisid dynasty in battle, taking Tlemcen and Fès.
The Fatimids create a new capital in Ifriqiya, Mahdiya.
June 21 – Ahmad ibn Fadlan is sent from Baghdad to Almış, first Muslim elteber (client ruler) of Volga Bulgaria, on behalf of the Abbasid Caliph al-Muqtadir.
The Later Liang Dynasty of China reports that all "barbarian" tribes have been pacified by the Khitan.
Treaty of Bonn: East France and West France recognize each other.
Ludmila of Bohemia is murdered at the command of her daughter-in-law at Tetin.
Henry the Fowler invades Bavaria, and obtains fealty from Arnulf the Bad.
A Hungarian army, led by Dursac (Tarhos?) and Bogát, defeats an army of insurgents, who planned to overthrow their ally, Italian emperor Berengar of Friuli, at Brescia.
October 27 – Emperor Shizong of Later Zhou
Edmund I of England (d. 946)
Ja'far ibn al-Furat, Ikhshidid and Fatimid vizier (d. 1001)
Abe no Seimei, Japanese cosmologist (d. 1005)
Ōnakatomi no Yoshinobu, Heian period waka poet and Japanese nobleman
Shaykh Syed Mir Mirak Andrabi, fifth generation of Sadaat-Andrabia in the Kashmir Valley
September 15 – Ludmila of Bohemia, ruler of Bohemia
Ragnall ua Ímair, Norse-Gael king of Northumbria
Richard, Duke of Burgundy (b. c. 867)
Vratislaus I of Bohemia (b. c. 915)
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