The Evil Entity appears as the source of all of the evil, all of the curse, all of the criminals who dressed as monsters and the pointless monster attractions of Crystal Cove and as the lord of all evil Annunake, he is definately the main antagonist throughout the whole series. He has tried to manipulate many mystery solving groups to set him from his Crystal Sarcophagus. He was sent in there by the other good Annunaki, along with the other evil Annunake.
The Evil Entity being imprisoned in the crystal sarcophagus.
He was destroyed when Scooby took out the sarcophigus with the spear, creating a brand new reality