A magnum opus is a great achievement—usually a great work of art, and especially one that is the single greatest work of someone’s artistic career. The term comes from Latin, where it means, literally, great work. Because it is well established in English (having entered the language over two centuries ago), it does not need to be italicized in normal use.
For it was this last phase of the struggle, a literary one, which produced Augustine’s magnum opus, The City of God. [Politics and religion in the days of Augustine, Edward Frank Humphrey (1912)]
He might perhaps have applied himself to the production of other work if hismagnum opus were not always there as an obstacle and excuse. [The history of American painting, Samuel Isham (1916)]
We believe that “Porgy and Bess,” which the composer took most seriously as his magnum opus in the field of music drama, has been enormously overrated. [New York Times (1941)]
Back in the late 1940s Hemingway was rumored to be writing an epic tentatively titled Land, Sea And Air, which was to be his magnum opus. [Montreal Gazette (1970)]
Francis Ford Coppola’s extended new version of his 1979 magnum opus amplifies everything brilliant about it, and everything exasperating. [Guardian (2001)]
Wagner’s magnum opus, properly known as Der Ring des Nibelungen, depicts the beginning and end of the world. [The Australian (2012)]