A worm (that looks like a finger) that burrows into large birds that eat it and then the worm fuses with the bird to turn into it's adult stage where it can lay eggs that look like bird eggs.
Sometimes the eggs can end up inside other nests, including domesticated chicken nests where the the egg is taken away and eaten by a human but the worms is still alive so it fuses itself with the human and takes over the human's body.
Its screech can hyptonize the prey it hunts when it becomes an adult.