|Covid-19|Spring – Bardas, the regent of the Byzantine Empire, is murdered by Basil the Macedonian at Miletus, while conducting a large-scale expedition against the Saracen stronghold of Crete.
May 26 – Basil the Macedonian is crowned co-emperor of the Byzantine Empire, and is adopted by the much younger Michael III.
May 27 – King Ordoño I dies after a 16-year reign. He is succeeded by his son Alfonso III ("the Great"), as ruler of Asturias.
July 2 – Battle of Brissarthe: Frankish forces, led by Robert the Strong, are defeated by a joint Breton-Viking army.
Harald Fairhair wins a decisive battle, in his quest to become king of all of Norway.
Louis II, Holy Roman Emperor, defeats the Saracen invaders who are ravaging southern Italy.
The Great Heathen Army of the Vikings rides north to Northumbria. The Northumbrians are preoccupied with a civil war, and the Danes enter York unopposed.
October 17 – Caliph al-Musta'in is put to death, after a 4-year reign. He is succeeded by al-Mu'tazz, who becomes the youngest Abbasid caliph to assume power.
The Kharijite revolt against the Abbasid Caliphate begins in Al-Jazira (Upper Mesopotamia), which will last for 30 years.
Fujiwara no Yoshifusa becomes regent (sesshō) to assist the child emperor Seiwa, starting the Fujiwara regency.
Boris I, ruler (knyaz) of the Bulgarian Empire, sends a diplomatic mission, led by the Bulgarian nobleman Peter, to Rome, in an effort to renew ties with the West.
Pope Nicholas I forbids the use of torture, in prosecutions for witchcraft (approximate date).
September 19 – Leo VI, Byzantine emperor (d. 912)
Carloman II, king of the West Frankish Kingdom (approximate date)
Robert I, king of the West Frankish Kingdom (d. 923)
Savaric I, bishop of Mondoñedo (d. 877)
Yao Yi, chancellor of Later Tang (d. 940)
April 21 – Bardas, Byzantine chief minister and regent
May 27 – Ordoño I, king of Asturias
June 21 – Rodulf, Frankish archbishop
July 2 – Robert the Strong, Frankish nobleman
July 16 – Irmgard, Frankish abbess
October 17 – Al-Musta'in, Abbasid caliph
Adelaide of Tours, Frankish noblewoman
Al-Mu'ayyad, Abbasid prince
Charles the Child, king of Aquitaine
Eberhard, duke of Friuli
Emenon, Frankish nobleman
Hungerus Frisus, bishop of Utrecht
Linji Yixuan, Chinese monk and founder of the Linji school
Liudolf, duke of Saxony
Ranulf I, Frankish nobleman (b. 820)
Robert, Frankish nobleman (b. 834)
Rudolph, Frankish nobleman
Wang Shaoyi, general of the Tang Dynasty
Year 866 (DCCCLXVI) was a common year starting on Tuesday (link will display the full calendar) of the Julian calendar.