The Evil Syndicate was formed when Professor Finbarr Calamitous stole the plans of an inter-dimensional portal through a flea-bot on Jimmy Neutron's pet dog Goddard. He used the portal to get access to different worlds and drain their energy with the help of each world's villain. In response, Jimmy assembled each world's heroes to fight the syndicate and stop their draining. They passed through Amity Park, Bikini Bottom and Fairy World before learning of The Syndicate's true plan: to build a Doomsday Device that can destroy a world in a matter of seconds. Once the villains were defeated, Calamitous called them back to attack the heroes all at once. But, even together the heroes managed to defeat the villainous group and shut down the machine, sending their villains (each one trapped somehow) back to their home worlds.
The Second Syndicate (Globs of Doom)
After Calamitous decided to follow up other plans of World conquest, another set of villains decided to take over the Syndicate. The new Syndicate was apparently leaded by Technus while the other members were Calamitous's Daughter: Beautiful Gorgeous, and sorcerer Traloc, with Plankton being the only member from the previous Syndicate. During the morphoid invasion, the heroes decided to work together with the Syndicate. Zim and Dib also were willing to help, with Dib deciding to help the Syndicate. The Nicktoons combined might along with The Vessel of Portencia is enough to stun the Morphoid leader: Globulous Maximous. However, the villains kick all the heroes out of the Vessel, planning to use Globulous to their own evil ends. Globulous, however decides to become good and defeats the villains, leaving them stranded in the empty voids of space.
Members: Professor Finbarr Calamitous (Leader and founder), Beautiful Gorgeous
Dib from Invader Zim seems to be an interesting exception to the Evil Syndicate, as his motivations in his own series align him as more of a hero than a villain. But it is apparent that he only joined the Syndicate because Zim joined the heroes' side in Globs of Doom.