Boril was the son of a sister of three emperors (or tsars) of restored Bulgarian Empire, Peter II, Ivan Asen I and Kaloyan.

[2] Historian Alexandru Madgearu says most recorded variants of the name (Borilǔ, Burile, Borilus, Burillus) suggest that "it was pronounced Borilǎ, as in Romanian".

[5] The Latin Emperor of Constantinople, Henry, stated in one of his letters that Boril "had imposed his will by violence and usurped the imperial name and insignia".

[6] Henri de Valenciennes (who continued the chronicle of Geoffrey of Villehardouin after 1207) likewise described Boril as a renegade who unlawfully seized the imperial crown.

[14] While Boril was waging war against the Latins, Stefan Nemanjić invaded Macedonia and occupied the land between the Struma and Vardar rivers.

[19] The Greeks of Serres in the Kingdom of Thessalonica sent envoys to Boril's commander in Melnik, seeking his assistance against the Latins in early 1209.

[20] Boril concluded an alliance with Theodore I Laskaris, Emperor of Nicaea, and Michael I Komnenos Doukas, the Greek ruler of Epirus, against the Latins.

[21] Boril sent an army to a mountain pass to prepare an ambush for the Latin Emperor Henry who was returning from Thessaloniki to Constantinople in April 1211.

[25] John Fine says Boril held the Church synod to secure the support of the clergy for himself, because "popular dissatisfaction with his reign may still have existed" in 1211.

[16][27][28] The exact circumstances of the movement are uncertain because the only source of information about these events are in fragmentary data preserved in a Hungarian royal charter issued in 1250.

[28] Boril was unable to suppress the rebellion without external assistance, turning to Andrew II of Hungary, reminding him of "their reliable friendship".

[28][29] Andrew dispatched Joachim, Count of Hermannstadt (now Sibiu in Romania), to Bulgaria at the head of an army of Saxon, Vlach, Székely and Pecheneg troops.

[33] After lengthy negotiations, Henry married Boril's stepdaughter (whom modern historians wrongly call Maria) in late 1213 or early 1214.

[34] In early 1214, Boril offered the hand of his unnamed daughter to Andrew II of Hungary's son and heir, Béla.

[36] In an attempt to conquer new lands, Boril launched an invasion of Serbia, laying siege to Niš in 1214, aided by troops sent by Henry.

[37][39] Boril was deprived of his two principal allies by 1217, as Latin Emperor Henry died in July 1216, and Andrew II left Hungary to lead a crusade to the Holy Land in 1217; this position of weakness enabled his cousin, Ivan Asen, to invade Bulgaria.