Tervel was the emperor of Bulgaria in the beginning of the 8th century. Khan Tervel belonged to the Dulo clan and reigned for 21 years. After the Bulgarian army crushed the arabs at the siege for Constantinopole (718) Tervel was called the saviour of Europe.
Tervel is mentioned first in Byzantine sources in 704, when he was approached by the deposed and exiled Byzantine emperor Justinian II. Justinian acquired Tervel’s support for an attempted restoration to the Byzantine throne in exchange for friendship, gifts, and his daughter in marriage. With an army of 15,000 horsemen provided by Tervel, Justinian advanced on Constantinople and succeeded to enter the city in 705. The restored emperor executed his supplanters, the emperors Leontius and Tiberius III and many of their supporters. Justinian awarded Tervel with many gifts, the title of kaisar (Caesar), which made him second only to the emperor and the first foreign ruler in Byzantine history to receive such a title, and a territorial concession in Thrace – region called Zagora.
On 25 May 717 Leo III the Isaurian was crowned Emperor of Byzantium. During the summer of the same year the Arabs led by Maslama crossed the Dardanelles and besieged Constantinople with 200,000 men. According to Arab sources his fleet consisted of 2,500 ships.
Leo III asked Tervel for help and Tervel agreed. The first clash between the Bulgarian and the Arab troops ended with a Bulgarian victory. During the first stages of the siege the Bulgarians appeared in the Muslim rear and large part of their army was killed and the rest were trapped. The Arab troops built two trenches around their camp facing the Bulgarian army and the walls of the city. They persisted with the siege despite the severe winter with 100 days of snowfall. In the summer of 718 the Arabs engaged the Bulgarians in a decisive battle but suffered a crushing loose and had to leave the siege. According to Theophanes, the Bulgarians slaughtered some 22,000 Arabs in the battle, while Zigebert writes for 30,000. The Byzantine-Bulgarian victory of 718 and the victory of the Frankish king Charles Martel in the battle of Tours stopped the islam invasion in Europe. Khan Tervel was called the Saviour of Europe by his contemporaries