、。
Before that night, there had never been any ill feeling between them.
Literal
That night before, they [genitive-の] between [in-に] ill-feeling [topic-は] never existed-not.
A tight narrative sentence built on the emphatic negative 決して~ない ('never,' 'by no means'). 決して is an adverb that appears only with negative verbs and serves to strengthen the negation. ~以前 ('before X,' 'prior to X') is a slightly more formal/written alternative to ~前, often used when marking a past turning point in narrative prose. 悪感情 (あっかんじょう) is 'ill will,' 'bad feelings' — more formal than 悪い気持ち, and register-matched to the narrative tone. Note the は on 悪感情 is a contrastive is-not topic: 'as for ill feelings, there were none' — implicitly opening the door to 'but after that night, that changed.'