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Kenilworth Castle, whence besieged followers of Simon de Montfort issue the Dictum of Kenilworth in 1266, seeking peace with the king of England.
Year 1266 (MCCLXVI) was a common year starting on Friday (link will display the full calendar) of the Julian calendar.
- February 26 – Battle of Benevento: The army of Charles, Count of Anjou, defeats a combined German and Sicilian force led by King Manfred of Sicily. Manfred is killed in the battle, and Pope Clement IV invests Charles as king of Sicily and Naples.
- July – Mary de Ferrers is ordered to surrender land and Liverpool Castle to Edmund, second son of Henry III.
- October – In England, the Second Barons' War winds down, as supporters of the slain rebel leader Simon de Montfort make an offer of peace to the king, in the Dictum of Kenilworth; after slight modifications to the peace settlement, it is agreed to the following year.
- The war between Scotland and Norway ends, as King Alexander III of Scotland and King Magnus VI of Norway agree to the Treaty of Perth, which cedes the Western Isles and Isle of Man to Scotland, in exchange for a large monetary payment.
- January 11 – Swietopelk II, Duke of Pomerania
- February 12 – Amadeus of the Amidei, Italian saint
- February 26 – Manfred, King of Sicily (b. 1232)
- June 12 – Henry II, Prince of Anhalt-Aschersleben (b. 1215)
- August 4 – Eudes of Burgundy, Count of Nevers (b. 1230)
- September 20 – Jan Prandota, Bishop of Kraków
- October 21 – Birger Jarl, Swedish regent and founder of Stockholm (b. c. 1210)
- October 28 – Saint Arsenije I Sremac
- October 29 – Margaret of Austria, Queen of Bohemia (b. c. 1204)
- December – John of Ibelin, jurist of the Kingdom of Jerusalem (b. 1215)
- December 3 – Henry III the White, Duke of Wroclaw
- Berke, khan of the Golden Horde
- Hugh Bigod, Justiciar of England
- Ariq Böke, regent of the Mongol Empire
- Maol Choluim II, Earl of Fife
- Mo'ayyeduddin Urdi, Syrian astronomer and mathematician
- Nasir ud din Mahmud, sultan of Delhi (b. 1246)