posthumous work
Refers to a creative work (art, literature, music, etc.) left behind by someone who has died, especially one published or exhibited after their death.
この小説は彼の遺作です。
This novel is his posthumous work.
This novel became his posthumous work.
展覧会では画家の遺作が展示されている。
The painter's posthumous works are on display at the exhibition.
遺族が画家の遺作を発表した。
The family released the painter's posthumous work.
An exhibition of her posthumous works will be held next month.
遺品 means any belongings or personal effects left behind by someone who has died, whereas 遺作 refers specifically to a creative work they left unfinished or unpublished at death.
Compound of 遺 (い, 'leave behind') and 作 (さく, 'work'). The kanji combination directly conveys the meaning of a work left behind after death.