Baron Mordo was a student of the Ancient One. The Ancient One taught Mordo about the mystic arts and how to use sorcery. Mordo eventually grew resentful of the Ancient One's power and wanted to take his place. Mordo cast a spell to kill the Ancient One. However, Stephen Strange overheard Mordo talk about his plans and attempted to warn the Ancient One but Mordo used his magic to subdue Strange. Mordo then cast his spell to kill the Ancient One. However, the Ancient One already knew of Mordo's plan to kill him and was prepared to defend himself. The Ancient One fended off Mordo's attack and Mordo managed to escape. The Ancient One then made Stephen Strange his new pupil. Doctor Strange then spent the next several years training to battle Baron Mordo.
Wand of Watoomb
Mordo later became a servant to an evil otherworldly demon called Dormammu. Dormammu was trapped in an alternate dimension called the Dark Dimension and he tasked Mordo with freeing him. If Mordo were to accomplish this Dormammu would drain an entire universe of it's life force.
Mordo eventually learned that Doctor Strange had possession of the Wand of Watoomb which could be used to open a portal to other dimensions and bring Dormammu to Earth.
Mordo then set up a cult called the Center for Reunification. Mordo used his magic to trick people into believing that they had been reunited with lost loved ones. Mordo did this because Dormammu could only come to Earth if he was accepted by humans. Mordo even recruited Mary Jane Watson and brainwashed her.
Dormammu later learned of Mordo's failure and scolded him. Dormammu then demanded that Mordo get the Wand of Watoomb no matter what the cost.
Peter Parker later tried to rescue Mary Jane from the Center for Reunification. However, Mary Jane activated an alarm and the other cultists rushed in and subdued Peter. However, Peter was holding back because he did not want anyone to learn that he was Spider-Man. The cultists then brought Peter to Baron Mordo. Mordo used his magic to place Peter under his control and then Mordo ordered Peter and the other cultists to attack the Sanctum Sanctorum and steal the Wand of Watoomb. Doctor Strange was able to break Spider-Man free from Mordo's control. However, the other cultists managed to steal the Wand of Watoomb.
Mordo then used the Wand of Watoomb to open a portal to the Dark Dimension to bring Dormammu into our world. However, Doctor Strange, Wong, and Spider-Man arrived. Doctor Strange and Mordo then used their magic to battle each other. One of Mordo's spells backfired and it caused the ceiling of the Center for Reunification to fall and almost crush him. Mordo realized he was defeated and escaped. Doctor Strange then used the Wand of Watoomb to close the portal to the Dark Dimension.
Baron Mordo controls magic forces. He can separate his spirit self from his physical body and travel anywhere on Earth. This is known as an astral form.
Mordo can use his magic to mentally control others.
Baron Mordo can also hurl magical bolts of energy from his hands.
Mordo possibly has a vast degree of other magic abilities.
In the comics
Baron Karl Mordo was born in Varf Mandra, Transylvania, Romania and was the son of Baron Nikolai and Baroness Sara Mordo. He gained an interest in magic and the occult from his grandfather, Viscount Heinrich Krowler. Mordo wished to expand his knowledge so he traveled to the mountains of Tibet to find the fabled Ancient One.
When Mordo found the Ancient One, the Ancient One realized lust for power made him dangerous but chose to teach him anyway feeling that he could at least keep a watchful eye on him.
Dr. Stephen Strange eventually found the Ancient One and overheard Mordo talking about his plans to kill the Ancient One. Strange attempted to warn the Ancient One but Mordo used his magic to trap him. However, the Ancient One knew of Mordo's plan to kill him and was able to fend off Mordo's attack.
Mordo then became one of Doctor Strange's archenemies.