The chemicals created slug-like monsters

It had been a few months after that accident. The toxic materials in the waste caused some invertebrates that lived in that pond to mutate. They became some kind of large slug thing that can swim fast underwater and even leap to ambush their prey.

They were small enough to hide themselves in spaces between rocks or hole in the ground, and large enough to overpower small animals like rats individually. In large number they could join **** to take down larger animals

They can control the body of their prey after they defeated them. They can even assimilate and modify the body of other animal that they had invaded through their orifice.

However, the slugs had a peculiar appetite towards humans. They saw them as food, like everything else, but they also saw them as some other things. It was unclear what these were, but for some reason the slugs were more attracted to humans than other animals.

And now the river brought them to the city. A city full of sewers, vehicles, plumbing pipes, and dark alleys. A city full of humans.