In this world there is a giant cloud of dust permanently on top of this continent, it makes it hard for sunlight to reach said continent, also there are only two giant continents on each side of the world.
The creatures can take over other animals, this is the way they can survive, by using their new host to feed on other animals meat, including other animals possessed by this creature when food is scarce. Their host's body gets bigger, and overall the creature is dangerous independently of whatever animal it took over and when it is night time they become more dangerous. Most of them are concentrated in the dark continent, but over the years some have migrated to the other continent where humans live.
What I would like to know is how I could make them irrelevant without killing off all the planet. Also, they are not a main threat to the story, but they are a constant threat that can't be forgotten about.
In this world Humanity has access to steampunk-like technology.