The Book of Boril or Boril Synodic (Bulgarian: Борилов синодик) is a medieval Bulgarian book from the beginning of the 13th century.It is an important source for the history of the Bulgarian Empire.
The book was written in conjunction with the Synod of Tarnovo (1211) convoked by tsar Boril against the Bogomils in 1211. Later additions and editions were made, dated at the end of XIV c.
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Bogomil (Cyrillic: Богомил) was a 10th-century Bulgarian priest who was connected with the origins of Bogomilism. Bogomil is a Theophoric name consisting of Bog (God) and mil (dear) and means "[one who is] dear to god". He was declared a heresiarch by both the Catholic and Orthodox churches.
According to Cosmas the Priest, Bogomil first began to preach his beliefs in Bulgaria during the reign of Peter I of Bulgaria (927 to 969), which indicates that Cosmas must have been writing later than 969. As with Cosmas, the life of Bogomil is shrouded in mystery and what little is known of him comes from the sermons written against him. There is some uncertainty about his relationship to Jeremiah or whether they are the same person. The statement that Jeremiah was "a son (disciple) of Bogomil" may be an interpolation. His name is mentioned in the Book of Boril.
The Synod of Tarnovo was a council of the Bulgarian Church, at that time in union with Rome, held at Veliko Tarnovo in 1211. Emperor (or Tsar) Boril of Bulgaria convoked it to condemn Bogomilism. The decisions of the synod were included in a document, now known as the Book of Boril.