Ultima: Hatred of Nibiru

In this new epic adventure from the Ultima series, we learn of how Scooby Doo and Mystery Inc. joined the Society, culminating in a horrifying final battle against the Nibiru Entity, bonded to Professor Pericles, the 3 Werecats, and Sarah Ravencroft in order to create the most terrifying evil the Scooby Gang has ever faced off.

(Can someone expand on this idea for me?)