grudge of ten years' standing; harboring a grudge for many years
A set phrase describing a deep, long-lasting resentment. Often used adverbially with に or as a noun. The 'ten years' is not always literal; it emphasizes the enduring nature of the grudge.
He swore revenge with a grudge that had lasted ten years.
After harboring a grudge for ten years, the time finally came.
遺恨 alone means 'grudge' or 'resentment'. 遺恨十年 is a fixed phrase emphasizing the long duration of the grudge.
Compound of 遺恨 (いこん, 'grudge') and 十年 (じゅうねん, 'ten years'). The phrase is a set expression; the exact historical origin is uncertain.