The history book records how many villagers were massacred during that war.

massacred 是动词 massacre 的过去分词形式。massacre 来自法语，意思是"大规模屠杀、残杀"，通常指对无辜人群的残酷杀害。在句中用过去分词形式表示被动，说明村民是被屠杀的对象。