Complex sentence: He was the artistic director of the Festival de Musique d'Albi from the creation in 1974 to the end in 1990, which became one of the most important place for chamber music and chamber opera (Rameau, Dauvergne, Philidor, Mozart, Britten).
Split into simpler sentences: He was the artistic director of the Festival de Musique d'Albi from its creation in 1974 to its end in 1990. It had become one of the most important places for chamber music and chamber opera (Rameau, Dauvergne, Philidor, Mozart, Britten).