The long-standing grudge between them has finally been resolved.
Tips
memory
宿 (sù) here means 'old / long-held' - the same meaning as in 宿敌 (sùdí, old enemy) and 宿命 (sùmìng, predestined fate). Combined with 怨 (yuàn, resentment), 宿怨 is resentment that has 'lodged' in the heart for a long time.
register
Literary / written register, common in fiction, history, and journalism describing feuds, vendettas, and long-running disputes. The everyday spoken equivalent is 老仇 (lǎochóu) or 旧账 (jiùzhàng, 'old accounts').
