The Mulgwisin is the vengeful spirit of someone who drowned, bound by the logic of "substitution" — to pass on to the afterlife it must pull another person down in its place. It is said to seize people by the ankles in rivers and ponds and drag them under.
The horror lies in the chain by which an unresolved death summons another. It is the archetype of Korean waterside lore, where the everyday boundary of the water’s edge becomes the most dangerous of thresholds.