The name means 'false vines' for obvious reasons. These mushi live in forests and feed on sunlight. If they're trapped in a place without much sunlight they'll be weak and looking for a way to improve their chances of survival; they will often take over the bodies of animals and people who fall into the valley, using them to gather sunlight, including the bodies of the dead. These dead bodies don't do much; they simply sit around in the sunlight and let their host mushi absorb the light. I assume the affected person will eat and drink like normal when prompted, or they'd attract too much attention, as this animation can go on for months at a time, if not years.
A nisekazura in its host body.
Nisekazura in the sun.
When they are stronger they turn black, and when enough of them reach this point, they will gather together, form themselves into a bridge, and leave the valley. When they do, the dead people and animals they were using go back to being normal dead bodies. This bridge is strong enough for humans to walk on, but there's a catch--because of the properties of the nisekazura, if you walk forward on the bridge you're fine. A single step backward, however, causes the vines to break and you to fall through them, usually to your death.
Nisekazura
The name means 'false vines' for obvious reasons. These mushi live in forests and feed on sunlight. If they're trapped in a place without much sunlight they'll be weak and looking for a way to improve their chances of survival; they will often take over the bodies of animals and people who fall into the valley, using them to gather sunlight, including the bodies of the dead. These dead bodies don't do much; they simply sit around in the sunlight and let their host mushi absorb the light. I assume the affected person will eat and drink like normal when prompted, or they'd attract too much attention, as this animation can go on for months at a time, if not years.
A nisekazura in its host body.
Nisekazura in the sun.
When they are stronger they turn black, and when enough of them reach this point, they will gather together, form themselves into a bridge, and leave the valley. When they do, the dead people and animals they were using go back to being normal dead bodies. This bridge is strong enough for humans to walk on, but there's a catch--because of the properties of the nisekazura, if you walk forward on the bridge you're fine. A single step backward, however, causes the vines to break and you to fall through them, usually to your death.