magnum opus
US /ˌmæɡ.nəm ˈoʊ.pəs/
UK /ˌmæɡ.nəm ˈoʊ.pəs/
1.
a large and important work of art, music, or literature, especially one regarded as the most important work of an artist or writer.
:
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His latest novel is considered his magnum opus.
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The symphony is widely regarded as the composer's magnum opus.