|Covid-19|January 11 – Ex-emperor Staurakios, a son of Nikephoros I, dies of putrefaction in his wounds (see 811) in a monastery. He has reigned only two months and eight days, before being exiled by senior officials in Constantinople.
Emperor Michael I re-opens peace negotiations with the Franks, and recognises Charlemagne as emperor (basileus) of the Frankish Empire. In exchange for this recognition, Venice is returned to the Byzantine Empire.
Byzantine–Bulgarian War: The Bulgars, led by Krum, ruler (khan) of the Bulgarian Empire, launch an invasion against the Byzantines. They capture the fortress cities of Develt and Mesembria, near the Black Sea.
Charlemagne conquers Catalonia, as far south as the River Ebro and the Balearic Islands. The counties come under the rule of Bera, count of Barcelona. He signs a three-year peace treaty with the Caliphate of Córdoba.
Charlemagne issues the Capitulare de villis, concerning the rights of a feudal landholder and the services owed by his dependents. It contains also the names of some 89 plants, of which the most are used medically.
The Republic of Amalfi sends galleys to support the Byzantine general (strategos) of Sicily, Gregorio, against the Aghlabid invaders. It is one of the earliest evidences of the independence of the city.
King Sigered of Essex is reduced to the rank of duke, by his Mercian overlords.
Fourth Fitna: Forces loyal to al-Ma'mun, led by Tahir ibn Husayn, blockade Baghdad, which is loyal to al-Ma'mun's brother, Caliph al-Amin, and begin the year-long Siege of Baghdad.
The Chinese government takes over the issuing of paper bank drafts, the ancestor of paper money.
Domnall mac Ailpín, king of Scotland (d. 862)
Li Rong, prince of the Tang Dynasty (or 813)
Sugawara no Koreyoshi, Japanese nobleman (d. 880)
Wang Yuankui, general of the Tang Dynasty (d. 854)
Wen Tingyun, Chinese poet and lyricist (d. 870)
January 11 – Staurakios, Byzantine emperor
Abd al-Malik ibn Salih, Abbasid general (b. 750)
Candidus of Fulda, Benedictine scholar
Du You, chancellor of the Tang Dynasty (b. 735)
Flann mac Congalaig, king of Brega (Ireland)
Fujiwara no Uchimaro, Japanese nobleman (b. 756)
Hemming, king of Denmark
Ibrahim I, Muslim emir of the Aghlabids (b. 756)
Jeong, king of Balhae (Korea) (approximate date)
Li Ning, prince of the Tang Dynasty (b. 793)
Nikephoros, son of Constantine V (approximate date)
Tian Ji'an, general of the Tang Dynasty
William of Gellone, Frankish nobleman (or 814)
Year 812 (DCCCXII) was a leap year starting on Thursday (link will display the full calendar) of the Julian calendar.