What was the name of this childrens horror book series?

Ok so I don't remember much about this series, all I know is it was similar to goosebumps, it was a long series of stand alone stories, though there was a three parter of thicker books within the series that all involved the same group of kids who like fought these evil people and had some kind of demon imp or something on their side. The title of the last book in this three parter was 'The Last-' That's all I remember, theres one more word but I'm not sure what it was. Also the three big books usually had monster info pages in the back of the book describing the bad guys from that volume. That's all I remember, does anyone know what book series this was?