The Joining are an extraterrestrial, mental, technological, societal, militant manifestation with an as yet unexplained origin and determination to conquer and destroy Earth. They are reminiscent of Brainiac (an enemy of Superman), as they lust to learn everything there is to know and conquer and destroy Earth.
In the two part episode, The Joining, they attacked Earth, causing destruction everywhere, and Batman and his enemies even worked together to destroy them. The Joining were supposedly destroyed. However, they later returned to Earth, being a little sneakier, for the last time was apparently a test, and this time, they had help from Dr. Hugo Strange, who consumed the powers of the Justice League, and gave them to the Joining. The heroes got back their powers, and destroyed the member of the Joining holding them, and the Joining's operating base was destroyed, along with the rest of the army of Joining duplicates.