καταλάβῃ, [1 verse] (3rd sg aor subj) "Come upon" is from katalambano, which means "to seize", "to lay hold of," [later] "to arrive at a place", "to seize for oneself", "to be possessed [passive of persons]", "to overtake", "to seize with the mind", "to comprehend", "to catch", "to find on arrival", "to come up with", "to hold down", "to repress", "to get under", "to hold [breath", "to bind", "to compel", "to restrain", "to force", "to convict," and "to condemn."  - (CW) The Greek word translated as "come upon" in the "darkness come upon you" primarily means "to seize," though it has a lot of other uses as well. It indicates the used of force much more strongly than "comes up" indicates. Christ is saying that darkness and ignorance seizes or overtakes you.