Mordu is a sorcerer and a DC supervillain.
Appearing mysteriously on Zerox, the Sorcerer's planet, he joined their ranks and rose in power quickly until, in a coup, seized absolute power over the planet, stealing the power of the other leading sorcerers for himself. One who escaped his grasp there was an apprentice of his, White Witch.
Armed with this mystical might, he embarked on a series of conquests of nearby worlds, with his magic, and built a space empire using the technological might of the worlds he conquered combined with his sorcery (for example, just before a space battle every weapon in the enemy fleets would jam). This combination proved unstoppable, and he conquered over half the galaxy in short order, establishing a tyrannical grip. In these days he also came to be called the "Dark Lord". The worlds which remained outside his grasp lived in fear. Then at some point, it seemed his power, which had scaled upwards astronomically, wavered. He no longer conquered worlds wholesale, but instead picked and chose carefully, "as if selecting baubles from a jewelers tray".
One of the key worlds outside his grasp was earth. The Legion of Super-Heroes defended it, and when he set his sights on it, a conflict was inevitable. In a titanic struggle, the Legion defeated Mordru's space armada, but then he himself entered combat personally, appearing as a 100' giant (Mordru commonly used magic to increase his size). He defeated most of the legion, but was sealed in an airless vault at superspeed by Superboy and Mon-El, triggering his old phobia.
They sealed him in an airless vault, where he remained comatose, beneath the legion headquarters until one day, Shadow Lass, exploring the headquarters, stumbled upon his vault and opened it to see what was inside. Mon-El tried to seal the vault (and in the process of warning her told the backstory). But air had entered the vault, Mordru roused, and proved unstoppable. The Legion fled via a Time Bubble. This early event foreshadowed problems Mordru would have with Time and those who manipulated it. Because they didn't have time to adjust the time bubble, it went to its last setting, Smallville in Superboy's time. They figured Mordru would think to look for them there, so they hid the Time Bubble, and adopted secret identities in Smallville, assisted by Superboy as Clark Kent.
Mordru used his magic to track them down, possessing various townspeople including Lana Lang to act as his spies and agents. When she found the legion, she reported them, and he arrived. They fought again, but the Legion finally triumphed in battle. As always, the end result left him trapped, comatose, but not dead.
Mordru was next freed by the Dark Circle, who sought to use him as a tool in their plans to conquer the earth. He quickly gained the upper hand, and used the Circle in a lengthy plot against Earth and the Legion, this time hiding his role throughout. The Earthwar began with the Khund invading the United Planets, and as the Legion tried to resolve this they discovered the Khundian leader was a puppet being controlled from afar, they eventually worked their way back through the Dominators, the Dark Circle and finally, when they confronted the Circle, found Mordru running the show. In the final battle, the entire Legion (at that point numbering about 30) attacked him en masse, but he shrugged off their attacks, creating a sphere which would kill them. At the last second, Element Lad, started to turn the air around Mordru into soil, thus triggering his phobia again.
After his previous appearance, he fled to a backwater planet named Avalon and took the name Romdur (an anagram of Mordru). Star Boy used his mass-inducing power to collapse his castle, trapping him again.
When Darkseid was reborn in the 30th century, he sought to gain power by draining the power of others. He freed Mordru and before Mordru could react, sapped his mystical energy. Mordru was left powerless. When Darkseid was defeated, one can assume the power returned to Mordru.
It was revealed that originally Mordru was fated to rule the universe, his rise to power was unstoppable. Seeing this timeline, Glorith, a time manipulator, changed history several times to block his rise to ultimate power. By repeatedly tweaking history she was orchestrating a battle between the Legion and Mordru in which they would be evenly matched, destroy each other, and let her pick up the pieces as the only remaining power in the universe.
Ultra Boy however figured out her plan, and turned the tables on her, visiting Mordru in disguise and provoking him into attacking Glorith before she was ready (but still more powerful than he expected). The result was a stalemate which cost both Mordru and Glorith the bulk of their power. In this retcon, this explains the sudden drop in Mordru's meteoric rise to power referenced above. He was still powerful, but had to content himself with ruling what he had conquered thus far.
This left him roughly at the point where he was pre-v4. Defeated after one of his schemes, the Sorcerers on Zerox removed his magical might and were able to "cure" his mind (details are vague, he seemed simpleminded afterwards), and he lived a happy life first on Zerox, then on Tharn after Zerox was destroyed in the Magic Wars (pretty much the entire planet relocated there). He even married his old apprentice and later enemy (and Legionnaire) Mysa Nal, the White Witch. In the economic collapse of the Galaxy which played a central role of the V4 legion, the planet Tharn was undefended and facing devastation at the hands of the Khund. As none of their other sorcerers had ever proved effective at fighting things such as starships, the Sorcerer's council decided to restore Mordru's power to him, despite knowing its corrupting influence on him. In short order he defeated the Khundish threat and set himself up as Emperor. His power was still nowhere near his glory days, though easily enough to defeat a Green Lantern, Rond Vidar who showed up on Tharn. The Legion turned up looking for him, he seemed capable of taking them all on but backed off when Cosmic Boy pointed out that he couldn't afford to take them on and the United Planets, which would happen if there was an incident. He relented, releasing them and Vidar.
Mordru eventually allied with Glorith in an attempt to further manipulate history again. These attempts weakened time and played a role in Zero Hour crossover event, which seemingly completely erased previous Legion continuity.
He also played a role in the history of Gemworld, where he was known as Wrynn, and fought Amethyst, Princess of Gem World. He was trapped inside stone there for a long period of time, which lead to him developing taphephobia (a phobia of being buried alive), a weakness that was often his downfall thereafter.
Mordru was such a dominant figure in the 30th century that other would-be conquerors had to account for him in their plans, sometimes involving the Legion in their attempts to get at Mordru. Pulsar Stargrave masqueraded as Brainiac 5's father and sent him on a series of missions to gather artifacts aimed at defeating Mordru. He even enlisted the aid of the The Time Trapper, ordering him to kill the Legion to prove his worthiness as a capable assistant in a coming battle against Mordru. When the Legion prevailed, he attempted to use them for the same purpose.
Later, an alien sorcerer named Sden tried to trick the Legion into retrieving yet another artifact, again with the goal of toppling Mordru.
Mordru has been described as one of the most powerful sorcerors to ever exist. He established an empire covering a vast area of the Milky Way Galaxy, but all outside United Planets space. He lead his conquest between the 28th Century and 29th Century before finally being defeated and imprisoned in an airtight vault on the asteroid Yuen by his daughter Mysa, out of revenge he kills all her allies and ages her into an old woman.
In the 30th Century he was released from his prison during a fight between the new Legionnaires Sensor, Umbra and Magno and a Legion of Super-Rejects on the asteroid.
When free he tries to retrieve his talismans, absorbing the power of some of his other descendent's, and searching for the Emerald Eye of Ekron, which was his favorite and most powerful talisman, but at the time it was possessed Shrinking Violet at the time. When he first encountered the Legion of Super-Heroes, he defeated them easily and sent them away to serve as his heralds, destroying the planet Sklar in the process as a demonstration of his power.
When he found the Emerald Eye of Ekron, it resisted, until he was finally defeated by a Legionnaires alliance including Mysa who was deaged and depowered during the fight, the Workforce and the Uncanny Amazers and sealed in an airtight sphere. While the Elements of Disaster threated to revive him at one point, he did not appear again.
Legends of the Superheroes
Gabriel Dell portrayed Mordru in the 1979 live action miniseries Legends of the Superheroes. In this, he sings an evil version of the song "That's Entertainment!".
Mordru appears in Justice League Unlimited's Season 1 episode "The Greatest Story Never Told". He is depicted as he was in his Silver Age incarnation. As the episode focuses on the adventure of Booster Gold (a fame seeking hero), Mordru's backstory is not explored. However, his god-like power is noted as it takes the entire Justice League (with the exceptions of J'onn, Shayera, Flash, and Booster Gold) to confront him. Mordru decimated Metropolis in a fight that took up an entire night, and used his magic to fuse Superman and Batman into one person (which resembles Composite Superman with Wonder Woman's voice). Elongated Man delivers the blow that knocks him unconscious defeating him.
Legion of Super-Heroes
In the Legion of Super-Heroes episode "Child's Play", a wizard that resembles Mordru (voiced by Richard McGonagle) appears a member of the Sorcerer's World Council on Zarok. Openly disdainful of the "United Planets", he supports a war with Earth over a minor breach in etiquette. Mordru appears as the villain of the second season episode "Trials" voiced by James Ward). He attempts to drain the magic of the wizards on the Council so he can rule the planet Zarok. He was defeated by Zyx and the Legion.