The phrase "slay en masse" refers to the act of killing a large group of people at once. There are several synonyms for this term that are often used in historical contexts, including "massacre," "carnage," "rampage," and "bloodbath." These words all convey the idea of large-scale violence and death, and are often used to describe tragic events such as wars, genocides, and other acts of inhumanity. While they may be difficult to express, it is important to remember the victims of these atrocities and to use language that accurately represents the gravity of their suffering.