|Covid-19|Byzantine–Bulgarian War: Emperor Nikephoros I organises a new campaign against the Bulgarian Empire, gathering a expeditionary force (around 80,000 men) from all parts of the empire. He is accompanied by high-ranking officials and aristocrats, including his son Stauracius and brother-in-law Michael I Rangabe (both emperors later, for a while). Krum, ruler (khan) of Bulgaria, sends envoys to sue for peace. Nikephoros refuses to accept the terms and marches through the Balkan passes towards Pliska, the Bulgarian capital.
July 23 – Nikephoros I reaches Pliska, and destroys a Bulgarian army of 12,000 elite soldiers who guard the stronghold. Another hastily assembled relief force of 50,000 soldiers has a similar fate. The Byzantines capture the defenseless capital. Nikephoros plunders the city and captures Krum's treasury. He burns the countryside, slaughters sheep and pigs, as he pursues the retreating Bulgars south-west towards Serdica (modern-day Sofia).
July 26 – Battle of Vărbitsa Pass: Nikephoros I is trapped (probably in the Vărbitsa Pass) and defeated by the Bulgars, who use the tactics of ambush and surprise night attacks to immobilize the Byzantine forces. Nikephoros himself is killed; Krum has the emperor's head carried back in triumph on a pole, where it is cleaned out, lined with silver and made into a jeweled skull cup, which he allows his Slavic princes (archons) to drink from with him.
Stauracius is installed as emperor at Adrianople (first time a Byzantine emperor is crowned outside Constantinople). Because of a sword wound near his neck (during the Battle of Pliska), Stauracius is paralyzed. The imperial court is split between the noble factions of his wife Theophano and his sister Prokopia.
October 2 – Michael I is declared emperor of the Byzantine Empire; Stauracius is forced by senior officials to retire to a monastery.
Treaty of Heiligen: King Hemming of Denmark concludes a peace treaty with Emperor Charlemagne in present-day Rendsburg. The southern boundary of Denmark is established at the Eider River.
Fourth Fitna: Abbasid caliph al-Amin appoints Ali ibn Isa ibn Mahan as governor of Khurasan, in northeast Persia, and sends him with an army of 40,000 men against his half-brother al-Ma'mun. Ibn Mahan's army is defeated by a smaller army under Tahir ibn Husayn, at Rayy. During the fighthing Ali ibn Isa ibn Mahan is killed.
Abu Hatim al-Razi, Muslim hadith scholar (d. 890)
Basil I, emperor of the Byzantine Empire (d. 886)
Muhammad al-Jawad, ninth Twelver Shī'ah Imām (d. 835)
Ōe no Otondo, Japanese scholar (d. 877)
June 17 – Sakanoue no Tamuramaro, Japanese shogun (b. 758)
July 24 – Gao Ying, chancellor of the Tang Dynasty (b. 740)
July 26 – Nikephoros I, emperor of the Byzantine Empire
Ali ibn Isa ibn Mahan, Muslim military leader
Charles the Younger, son of Charlemagne
Li Fan, chancellor of the Tang Dynasty (b. 754)
Pei Ji, chancellor of the Tang Dynasty
Pepin the Hunchback, son of Charlemagne
Year 811 (DCCCXI) was a common year starting on Wednesday (link will display the full calendar) of the Julian calendar.