magnum opus
noun /ˌmæɡnəm ˈəʊpəs/
/ˌmæɡnəm ˈəʊpəs/
[singular] (from Latin)- a large and important work of art, literature or music, especially one that people think is the best work ever produced by that artist, writer, etc.
- The author produced his magnum opus just before he died.
Word Originlate 18th cent.: from Latin, ‘great work’.