magnum opus /(măg"nŭm ō"pŭs)/

mag·num o·pus

magnum opus

n.
  1. A great work of art or literature.
  2. The greatest work or achievement of a particular artist, writer, or other individual.
    Inspired by this milieu, [Max Stirner] wrote his magnum opus The Ego and Its Own, which was published in November 1844.