See his memoirs (tr. 1919, repr. 1970); study by J. Harding (1970).
It was first performed at the Opéra in Monte Carlo on April 1, 1922, nearly ten years after Massenet's death. Massenet had started to compose the piece in 1895 but shelved it and completed it clandestinely in the last years of his life. Amadis is one of three operas by Massenet to have premiered posthumously; the others are Panurge (1913) and Cléopâtre (1914).
The story takes place in ancient Brittany and concerns the brothers Amadis and Galaor, separated at birth. Amadis kills Galaor in the final scene in a duel over the princess Floriane. When Amadis discovers magic stones around Galaor's neck identical to those given to him and his long lost brother by their dying mother, he realizes it is his brother he has killed.