Mongul was the unquestioned ruler of War World, an oppressed planet where alien gladiators battled endlessly in violent public exhibitions.
When J'onn J'onzz and Superman were incapacitated by a freak explosion in space, they were collected by the minions of Mongul. Mongul presided over the gladiatorial combat of War World, an ongoing series of deathmatches created to distract Mongul's subjects from the poverty and oppression of their daily lives.
Superman was recognized as a Kryptonian, and Mongul sent the Man of Steel into combat against War World's undefeated champion, Draaga. The victorious Superman refused to kill Draaga in defiance of War World tradition, which enraged Mongul and threatened his position as absolute ruler in the wake of Superman's overwhelming public support.
To counter this, Mongul arranged a personal match with Superman, and threatened to destroy Draaga's planet (since Krypton was unavailable) if Superman did not lose purposely. However, the planet-killing weapon was destroyed before Mongul could claim victory.
Mongul was finally defeated by the returning Draaga, who at Superman's urging would take the throne of War World and hopefully usher the populace into a more peaceful and prosperous era.
It is not known what happened to Mongul immediately following these events.
Mongul returned some years later on the occasion of Superman's birthday. The former conqueror made a "present" of an alien parasite called the Black Mercy. The unsuspecting Superman was attacked by the parasite, which paralyzed him immediately and trapped him into a dream-induced state. Wonder Woman and Batman had arrived at Superman's Arctic fortress to celebrate and discovered the catatonic hero. Mongul stepped out of the shadows and explained the gift and its purpose: With Superman out of the picture, Mongul planned on conquering Earth, destroying its heroes, and creating a new War World. As Wonder Woman attacked him with little effect, it became obvious that the alien warrior's threat was very serious.
After Superman was freed, he tore into Mongul and overpowered him, but just as he was about to finish him, he was suddenly distracted by an image of Krypton. Mongul used this distraction to gain the advantage, but just as he was about to crush Superman's skull, Wonder Woman threw the Black Mercy down upon Mongul, incapacitating him with his own weapon.
Mongul's final fate is unrevealed. Though paralyzed by the Black Mercy, he is still alive and was presumably imprisoned, living indefinitely within his dream world.
Powers and abilities
Mongul is a cunning strategist capable of managing an entire war-based society, and has approximately Superman-levels of strength and durability. In a fight, Mongul was able to completely dominate Wonder Woman, and Wonder Woman even injured her hands punching him several times in the jaw. During their fight, Mongul even stated that he considered Superman to be the only real threat to his plans for Earth.
In addition, he has access to highly advanced alien technologies. Using psychology and brute force, he has intimidated entire planets into ruin.