The Dark Heart was a highly advanced alien construct capable of endlessly recreating itself via the use of nanotechnology.
The "Dark Heart" was created 50,000 years ago as a final weapon in the battle between two far-off planets. It successfully consumed its creators' enemies—and promptly returned to feast upon its creators themselves. Afterwards, it endlessly continued the cycle of consuming planets, reproducing then moving onto the next planet across the galaxy. 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.
When Brainiac revealed himself inside Lex Luthor's body, it absorbed the nanotechnology of the Dark Heart. It skillfully mastered the alien device, but was eventually defeated by Flash, who overloaded the technology by applying the Speed Force.
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 chip stored in its main core to calculate all its planning, including combat strategy and nanotech control.