opus magnum
/ˌəʊpəs ˈmaɡnəm/noun
A significant achievement; specifically a large and important work of art, music, or literature, especially one regarded as the most important work of a particular artist.
Origin
Early 18th century; earliest use found in Jonathan Swift (1667–1745), writer and dean of St Patrick's Cathedral, Dublin. From classical Latin opus magnum great labour (Caesar, of contending with a storm at sea) from opus work + magnum, neuter of magnus great. Compare magnum est…opus the labour is great, used by Cicero of an orator's material.