In the Matter of Britain, the Civil War of the Two Brothers was a war between Mempricius and his brother, Malin, the sons of Maddan, high king of the Britons. After Maddan's death, a family feud broke out between the brothers over who would dominate Britain.

The war lasted a number of years until Mempricius called a conference with his brother and other delegates to end the war between the two brothers. Once there, Mempricius killed Malin and took the throne of the Britons for himself, thus ending the war.