This means to make sure that you do something. Alternate translation: "be sure to" or "make sure that you" (See: [[rc://en/ta/man/translate/figs-idiom]])
Here the king and his people being killed is describes as if they were "swallowed" by their enemy. Alternate translation: "the king will be killed" (See: [[rc://en/ta/man/translate/figs-metaphor]])