Mongul the Elder first appeared in DC Comics Present #27 and is created by Jim Starlin and Len Wein.
Not to be confused with Mongul.
Mongul was the ruler of his people, until a revolution occurred and he was forced into exile. In Mongul's first appearance he kidnapped Lois Lane, Jimmy Olsen, and Steve Lombard and threatened to kill them unless Superman gave him the key to Warworld. He retrieved the key and mentally linked to Warworld's controls. He attempted to kill Superman and Supergirl, but he was ultimately rendered unconscious due to the mental strain of hooking his mind up to Warworld. Mongul manages to escape before Warworld is destroyed.
Mongul then attempts to conquer Prince Gavyn's homeworld, Throneworld. Mongul kills Prince Gavyn's sister and forces the prince's love to marry him so he can take the thrown. He blackmails the resident planets to submit to him with Throneworld's planet-destroying weapon. Superman arrives and battles Mongul while in the meantime Gavyn shuts down the weapon, forcing Mongul to retreat. After this defeat Mongul desires revenger against Kal-El. He kills a Controller and steals the Sun-Eater to devour Earth's sun. The Legion of Superheroes and the Justice League battled Mongul, but it was futile until Superman finally defeated Mongul while the Legion of Superheroes destroyed the Sun-Eater.
Mongul then attacks the kryptonian on is birthday and ensnares him with a Black Mercy. In the end he is a victim himself of the Black Mercy and dreams of a universe where he is its ruler.
In the Modern Age Mongul was reintroduced as the ruler of Warworld and entertained himself and his citizens with gladiator like games.
Mongul captures Superman to participate in his gladiator games, but Superman joins forces with another warrior named Draaga causing Mongul to flee. Mongul was then forced by Cyborg Superman to serve him and turn Earth into another Warworld. In the process of making Earth another Warlworld Coast City was destroyed which lead to Hal Jordan helping Superman and his allies defeat Mongul.
He was arrested for his crimes and is imprisoned in a jail for intergalactic criminals, but broke out during a riot. He planned his revenge against Green Lantern and found that the one he was facing was not Jordan, but actually a new Lantern named Kyle Rayner. Rayner defeats Mongul and he is re-imprisoned.
Mogul once again breaks out of prison and is teleported to a planet and saved. In gratitude Mongul takes over the planet and the inhabitants die of a virus leaving Mongul one of last inhabitant of the planet. He then finds two alien babies who are immune to the virus; these children grow up to be Mongal and Mongul II.
Mongul returns to Earth only to be defeated by Wally West. During Underworld Unleashed he is offered a deal by Neron, but Mongul refuses the deal and threatens Neron. In the end Neron kills Mongul.
New 52: Villains Month
Mongul is given his own issue during the New 52's villains month. This issue shows us the new look and whereabouts of Mongul. The issue starts off with a foreign police force surrounding Warworld demanding its surrender. The police chief who was at first demanding surrender is then transported onto WarWorld and is immediately exposed to the brutal gladiatorial arena of WarWorld. After this traumatizing experience he is then introduced to Mongul who at first appears friendly yet intimidating. After giving a brief tour of WarWorld, Mongul describes the process of how the commanders planet is being destroyed and the population being abducted by robots sent from WarWorld as they speak. The police commander is horrified by the turn of events that have taken place, and is then brought to a strange room were there are hundreds of heads on pikes, the commander questions Mongul about the room and Mongul explains to him that "all these heads demanded my surrender", then Mongul rips the police commander's head off. The end of the issue hints towards Mongul's future in the New 52 when his scout report that Warworld is coming into a sector protected by the Green Lantern Corps.
Mongul the Elder had Physical Strength greater than Superman; in fact Superman was only able to beat him in a fight once, while Mongul himself overwhelmed Superman numerous times. Mongul has also beaten Hal Jordan, Supergirl, & Martian Manhunter with just pure brute force, and broke out of the hardest material in the universe with ease.
Mongul The Elder was almost invulnerable to nearly all types of harm. Wonder Woman hurt her own hands just punching the War Lord's face.
Mongul also had an energy cannon on his chest, and was able to to fire energy from his eyes and hands. In addition, Mongul had limited telepathy and telekinesis.
He had the ability to teleport, though for a long time was unknown whether it was an actual ability or own caused by technology, until after a battle with Superman & Starman damaged his costume; he managed to get away from them by teleporting.
He commanded numerous arrays of technology including dimensional inversion cubes, and the planet-sized engine of destruction War World.
Mongul Sr. was also a very intelligent and cunning foe, but even more dangerous than either his intellect or physical stats was his complete and total lack of respect for life. He had little to no respect for other beings' lives and would do anything to accomplish his goals, even destroying entire worlds.
Mongul Sr. was also a superb combatant able to take on entire teams of heroes alone and still come out on top.
Mongul appears in the Justice League episode "War World" where he is voiced by Eric Roberts. He forcefully coerces Superman into fighting for him in Gladiator-style matches, providing entertainment for the masses. Ultimately, however, he is defeated, and consequently humiliated, in single-combat by Superman and Draaga.
Justice League Unlimited
Mongul traps Superman in The Fortress of Solitude on Superman's birthday using Black Mercy plant. His Voice work was once again provided for by Eric Roberts from Justice League, Mongul returns to claim revenge on Superman by killing him and taking the Earth under his tyrannical rule. After totally beating down Wonder Woman effortlessly, and tossing Batman aside like a ragdoll, Mongul got into a brutal battle with the Man of Steel, gaining an upper hand he was going to finish off Superman until Wonder Woman tossed the Black Mercy onto him trapping him in his own dark twisted Fantasies.