The Nurikabe suddenly appears before a night traveler and becomes an invisible wall that blocks the way. Go around it and it stretches on without end; you cannot pass over or under, so you are brought to a halt where you stand.
Its core is that without harming you directly, it traps you in the helplessness of being unable to go on. With a remedy passed down — tap the wall low with a stick to make it vanish — it is a case where the very state of being unable to proceed becomes the creature.