Ma Da is said to be the soul of someone who drowned. It is bound to the place where the death occurred — a small pond, a lake, a river or a stream — and is pictured as a bloated corpse with water weeds tangled in its hair. Because the death was one of torment rather than peace, the soul cannot move on and keeps to the water’s edge.
Its central motif is the substitution of a life, called thế mạng in Vietnamese. The drowned soul is believed unable to pass on until it drowns a living person to take its place. Accounts describe an unseen grip seizing a swimmer’s ankle and dragging it under, or a current that was not there before pulling a wader into the deep. When the new victim’s ghost inherits the old one’s place, the cycle of drawing others down begins again at the same spot.
The belief works as a folk way to explain and warn against the drownings that recur at certain stretches of water. Ma Da is received less as a monster than as a pitiable, unquiet soul. People therefore set offerings at the water’s edge to console it, hoping the trapped spirit can be released without a living person being taken in exchange.