The phrase "great slaughter" means a large-scale killing or massacre of a significant number of people or animals. It refers to a violent event where many lives are lost. Full definition
Jerusalem, Bethlehem, and other sacred places were taken, with great slaughter of Moslems, and the crusading Kingdom of Jerusalem was set up. (religion-online.org)
The First Crusade captured Antioch in 1098, with great slaughter, and in the following year took Jerusalem, killing many of its inhabitants. (religion-online.org)
33 He struck them from Aroer until you come to Minnith, even twenty cities, and to Abelcheramim, with a very great slaughter. (religion.blogs.cnn.com)