ねばならない/ねばならぬ/ねば

「ねばならない/ねばならぬ/ねば」express the idea of "must" or "have to" do something.
They are synonymous with 「なければならない」and carry a sense of obligation, compulsion, or command, but due to their stiffness and old-fashioned nature, they are hardly used in everyday conversation. Instead, they are mainly found in movies, old literature, or settings from the past.
「ねばならぬ」is an older form of expression, and 「ねば」is a shortened form.