The Ethricole are the main antaganist throughout the series, although it doesnt mention them until more than halfway through the series.
Echthroi; What are they?Edit
The Echthroi are shadow beings that can take over a persons body, but can also manifest themselves into different shapes. For instance, Loki takes the form of a large blackbird and Grimalikin takes the form of a cheshire cat. When they do take over a host, they are reffered to as Lhigor.