역작이 · 역작을
work of great effort, labor of love, masterpiece
Example
이 소설은 작가가 10년 동안 공들여 쓴 역작입니다.
This novel is a labor of love the author spent ten years writing.
Word parts
역 effort+작 work
Literally 'strength work' — made with tremendous effort.
Good to know
Pronounced [역짝]; often paired with 심혈을 기울인 'poured heart and soul into.'