magnum opus
noun [ S ] formal uk/ˌmæɡ.nəm ˈəʊ.pəs/ us/ˌmæɡ.nəm ˈoʊ.pəs/
（作家或艺术家的）代表作，力作
the most important piece of work done by a writer or artist
Picasso's Guernica is considered by many to be his magnum opus. 许多人认为《格尔尼卡》是毕加索的代表作。