615 Explained

Year 615 (DCXV) was a common year starting on Wednesday of the Julian calendar. The denomination 615 for this year has been used since the early medieval period, when the Anno Domini calendar era became the prevalent method in Europe for naming years.

Events

The Balkans are freely overrun by the Slavs, who settle in large numbers in what is now Bulgaria, Serbia, North Macedonia and parts of Greece. The western territories of modern-day Yugoslavia (Bosnia, Croatia and Dalmatia) suffer raids from the Avars, who settle in this region.
The Slavs under Chatzon attack in longboats along the coasts of Thessaly, western Anatolia, and various Greek islands. They besiege the Byzantine city of Thessaloniki in a combined land and sea attack. The Slavs with their families encamp before the city walls.
The city of Epidaurus (Dalmatia) is destroyed by the Avars and Slavic invaders. The Illyrian refugees flee to the nearby island Laus, where they found Dubrovnik (Ragusa). The islands of Rab, Krk and Cres become major food suppliers for the surviving cities of the mainland.
Eleutherius succeeds John I as exarch of Ravenna. He persecutes those implicated in the murder of John and the judges of the State. After making a courtesy visit to Pope Adeodatus I, Eleutherius captures Naples and kills the rebel leader John of Conza.^[1]

Bangor Massacre: The Anglo-Saxons, under King Æthelfrith of Northumbria, after defeating the Kingdom of Powys, finally reach the Irish Sea and massacre 1,200 Christian monks at their monastery, in Bangor (Wales).^[2]
Edwin of Northumbria takes refuge in East Anglia; he marries Cwenburga, daughter of king Cearl of Mercia (according to the Historia ecclesiastica gentis Anglorum).

July 29 - Queen Sak Kʼukʼ is succeeded by her son Pacal the Great, as ruler of the Maya city state Palenque (Mexico). He begins a building program at his capital, that produces some of Maya civilization's finest art and architecture.

May 25 - Pope Boniface IV dies after a 7-year reign, in which he has converted the Pantheon into the Church of "Santa Maria Rotonda". He is succeeded by Adeodatus I (also known as Deusdedit) as the 68th pope of Rome.
Several of Muhammad's followers begin to emigrate to the Aksumite Empire. They found a small colony there, under the protection of the Christian Ethiopian emperor Așhama ibn Abjar.^[3] ^[4]

Raymond Davis (translator), "The Book of Pontiffs (Liber Pontificalis)", first edition (Liverpool: University Press, 1989, p. 63
http://www.genuki.org.uk/big/wal/FLN/Bangor/ St Dunawd
Alford Welch, "Muhammad", Encyclopaedia of Islam
An Introduction to the Quran (1895), p. 185