- You may be looking for the Justice League Unlimited episode "Dark Heart", or the Heart of Darkness.
The Dark Heart was a highly advanced alien construct capable of endlessly recreating itself via the use of nanotechnology.
History[]
The "Dark Heart" was created 50,000 years ago as a weapon in the battle between two far-off planets (possibly across a galaxy). It successfully consumed its creators' enemies—and promptly returned to feast upon its creators themselves. Afterwards, it (possibly) endlessly continued the cycle of consuming planets, reproducing then moving onto the next planet. Eventually, the Dark Heart found its way to Earth, landing on a mesa near Goldhanger, Nevada.
Upon its discovery, an Omega level threat was immediately issued, but strike groups from Nellis Air Force Base and a large number of Justice League members could only barely manage to contain it. Not until Atom found its weakness—a central power core—could the alien machine be defeated. Atom traveled inside the Dark Heart, and successfully gave it a "heart attack", rendering the Heart and its constructs motionless.
The remains were confiscated by General Eiling, and eventually found their way to a Cadmus storing facility. Research on it by Cadmus scientists proved to be fruitless, however.
After Brainiac revealed himself inside Lex Luthor's body, he absorbed the nanotechnology of the Dark Heart. He skilfully mastered the alien device, but was eventually defeated by the Flash, who destroyed the technology by delivering hyperfast punches to "Lexiac's" body.
Powers and abilities[]
The Dark Heart could convert any raw material it touched into a viscous liquid that served as the base for several independent constructs. If any of the constructs were damaged or destroyed, its fragments would instinctively recreate the original machine and a copy created from surrounding matter. Only one weakness plagued the formidable technology: it required a central processing unit stored in its main core to calculate all its planning, including combat strategy and nanotech control.