The negotiations were long and eventful, with several unexpected breakthroughs.

eventful 是由名词 event（事件）加上后缀 -ful（充满...的）构成的形容词，字面意思是"充满事件的"，引申为"多事的、多变的、重要的"。 在句中形容谈判过程发生了许多重要事件和变化。