In Latin, nex means "murder" or "violent death." Sacramentum means an oath (or a legal deposit or religious rite). The Latin meaning of the phrase "nex sacramentum" is unclear, since both words are nouns in the nominative case. "Death oath" would be expressed something like sacramentum necis wiki上查到的,好像就是编剧自己造出来的词,不存在的