He harbored a grudge against his former boss.
Deep-seated resentment remains between the two countries.
彼は遺恨を晴らすために復讐を誓った。
He swore revenge to settle the grudge.
A long-standing grudge still lingers between the two families.
He finally got the chance to settle an old score.
怨念 carries a stronger supernatural or obsessive nuance, like a vengeful spirit's grudge, whereas 遺恨 is about human historical or political resentment.
敵意 means open hostility or animosity toward someone, while 遺恨 specifically refers to a grudge that lingers from past wrongs.
deep-seated grudge; rancor; spite
grudge of ten years' standing; harboring a grudge for many years
grudge match
Compound of 遺 (leave behind) and 恨 (resentment), literally 'left-behind resentment'. The exact historical derivation is uncertain, but the spelling is conventionally associated with a grudge that persists over time.