Baron Mordo appeared in 1963 Marvel Comics called Strange Tales.
In the aftermath of the First World War, Baron Nikolai Mordo of Transylvania grew angry at the modern world. Desiring for his native land to return to its former glory, he asked Miarka, Witch Queen of the Romani to teach him the ways of black magic so that he may achieve his goal. Viscount Heinrich Krowler of Bavaria, himself seeking revenge upon the Allied powers, learned of the Baron's emerging skill as a magician. Krowler had already reached the limits of his mystical potential, so he attempted a spell of transference to steal Nikolai's abilities. Unfortunately for Krowler, the spell could only work on a member of his own bloodline. Changing plans, Krowler sent his daughter, the Baroness Sara, to Transylvania to marry Nikolai. Nine months after her wedding, Sara gave birth to a son: Karl Amadeus Mordo.
Karl Mordo's childhood was a lonely one. He was often ignored by both his mother, who considered him to be little more than an element of Krowler's scheme, and his father, who was too busy pursuing his own goals of Carpathian supremacy. Fearing the Baron's plans would interfere with his own, Krowler ordered Sara to kill Nikolai. Following the murder, Sara and her son moved from their castle in Varf Mandra, Transylvania back to Bavaria. Krowler began to teach his grandson in the ways of black magic. Mordo became an apt pupil. One night, Mordo learned of his father's murder as well as his grandfather's plans for him. Alone in his room, Mordo swore to avenge his father, as well as carry out his plan of a perfect feudal world, with Karl ruling over it as the second Baron Mordo.
At the age of 18, Krowler sent Mordo to the mountains of Tibet to find the fabled Ancient One.
Mordo began his tutelage under the Ancient One as early as the 1930's. Already an adult, residing in the Ancient One's lair brought him reduced aging. The Ancient One realized that Mordo's lust for power made him dangerous but chose to teach him anyway feeling that he could at least keep a watchful eye on him. While poring over old scrolls and parchments, Mordo learned of an entity called the Dread Dormammu. The Ancient One ordered Mordo to not seek Dormammu and forbade him from discussing the matter any further. One night, Mordo defied the Ancient One and made contact with Dormammu. Through the spell of transference, Krowler himself learned how to summon Dormammu. In 1943, Dormammu possessed Krowler's body but he was eventually driven out by a time traveling Doctor Strange, leaving Krowler's mind and body destroyed. Mordo learned of his grandfather's condition and, after placing a spell on the Ancient One allowing him to leave undetected, returned to Bavaria. Mordo captured his mother and grandfather and sacrificed them both to Dormammu (after some deliberation as to just which demon he would sacrifice them to).
The Ancient One knew of Mordo's actions and resolved to train a sorcerer of great power to stand against Mordo. Decades later, Stephen Strange arrived in Tibet seeking the Ancient One.
Mordo was immediately resentful of Stephen Strange when he began studying under the Ancient One and magically restrained him from speaking when Strange accidentally walked in on Mordo's plot. The plot failed, Strange became the Ancient One's disciple, and Mordo eventually left.
Mordo continued his studies and became a fierce rival of Doctor Strange. They battled many times, in an early account Baron Mordo used his powers to try and eliminate the Ancient One. Baron Mordo controlled the Ancient One's assistant to poison the Ancient One. Sensing his master in trouble, Strange battled Mordo in his ectoplasmic form, preventing Mordo from ending the Ancient One's life. Strange was soon targeted by Mordo directly, who lured Strange to England to aid mystic Sir Clive Bentley. Strange only found Clive's daughter Victoria Bentley, a latent mystic herself, and fell into Mordo's trap. When the Baron attacked Strange, he was still no match for Stephen's superior mystical abilities and was forced to flee the scene. Mordo soon struck again, this time tricking Strange into leaving his body in his spirit form. Knowing that if Strange was separated from his mortal body for more than 24 hours he would die, Mordo trapped his ectoplasmic form. Seeking the Ancient One's aid, Strange was able to recover a mystical ring that allowed him to break Mordo's spells and return to his mortal body, banishing Mordo once again.
Mordo created a miniature replica of the Sanctum and planned to transport Strange and his home into an unknown dimension. Seeking the spell's source, Dr. Strange travelled in ethereal form to the home of Baron Mordo where he fell into another trap, a bottle like prison. Mordo arrived at the Ancient One's house only to discover that Strange had reached there before him. Mordo's magical prison had no floor. By descending downwards through the Earth, Strange had eventually weakened the spell enough to break free. In the following magical battle Dr. Strange's good powers defeated Mordo's evil ones. Mordo stole Strange's body when he was in the Astral Plane, in hopes that Strange's body would die without it's soul. He then sent an ectoplasmic warrior to attack him and put his body on display in a wax museum. They battled in the Astral Plane, and Strange won and then trapped him there, and placed Mordo's body on display in the wax museum. Mordo kidnapped the Ancient One. Strange searched the world over trying to find the Ancient One while simultaneously trying to prevent Mordo from capturing him. Finding and freeing the Ancient One in ancient Aztec ruins, Dr. Strange confronted Mordo and defeated him in a mystic duel.