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Mordru, aka Mordru the Merciless, aka "The Dark Lord" is one of the principal villains in Legion History. He was created by Jim Shooter.
He first appeared in Adventure Comics 369, in which a backstory was told of his origin.
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, Mysa Nal aka the 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 of when he would come for them, and dared not plan to free the enslaved worlds lest they anger Mordru. Then at some point, it seemed his power, which had scaled upwards astronomically, wavered. As Shooter put it, 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. But air had entered the vault, Mordru roused, and proved unstoppable even to Superboy and Mon-el. 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 immediately realized it was the one era Mordru would likely come looking for Superboy, but instead of going to another time, they hid the Time Bubble, and adopted secret identities in Smallville, assisted by Superboy as Clark Kent. Mon-El resumed his Bob Cobb identity he had used when he first visited Smallville as a brush salesman, and new identities were found for the others, including Shadow Lass who had to use make-up to cover his blue skin.
Sure enough, 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.
This established the classic Mordru story cycle. Every once in awhile, something would again free Mordru, he would begin his machinations against the Legion, with whom he became increasingly obsessed, and they would again defeat him, leaving him trapped until a writer sought to revive him again...
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 (Federation like organization which Earth was a part), 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, a transmuter, started to turn the air around him 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). This was a weak appearance, not noteworthy. 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.
V4 (aka "Five Years later") Legion version
In the V4 legion, 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 which wiped out the V4 legion continuity.