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.
Venom & Carnage
Baron Mordo later returned with another plan to free Dormammu from his dimension. Using the alias Ardon Broom (which is an an anagram of Baron Mordo), Baron Mordo started a technology company which supplied Stark Enterprises with equipment to build a machine that could open portals to other dimensions. This device was called the Inter-Dimensional Probe. Dormammu then had Mordo break into a control room in Stark Enterprises and use a computer to bring the John Jameson Probe back to Earth. Mordo did this because they were planning to re-bond the symbiote to Eddie Brock.
Mordo later sent his astral for to the Ravencroft Institute for the Criminally Insane and spoke with Eddie Brock. Mordo told Brock that his master, Dormammu, had the power to give him back the symbiote and he had to do was swear to serve Dormammu forever. Brock agreed to this and Mordo informed Dormammu.
When the symbiote got back to Earth, Mordo used his sorcery to hypnotize a woman into bonding to the symbiote and deliver it to Eddie Brock at Ravencroft. The symbiote then bonded to Eddie Brock turning him into Venom again. After escaping Ravencroft, Venom went to Dormammu's lair. There Dormammu explained to Venom how Mordo was able to bring the symbiote back to Earth. Dormammu then had Venom go to Stark Enterprises to retrieve the Inter-Dimensional Probe.
When Venom got to Stark Enterprises he fought Spider-Man and War Machine for the Inter-Dimensional Probe. Mordo was at Stark Enterprises while this was going on and secretly watched them fight. An image of Dormammu then appeared to Baron Mordo and Mordo told him that he feared the Venom would be defeated and they could lose the Inter-Dimensional Probe. However, Dormammu replied that he had him bring the symbiote back to Earth not only for it's strength but also because it was about to reproduce. Dormammu told Mordo that this other symbiote could help Venom.
A short while later Mordo had a man bring the new symbiote to Ravencroft and deliver it to Cletus Kasady. Mordo's astral form then appeared to Kasady and told him that the symbiote was a gift from his master, Dormammu. The symbiote then bonded to Kasady and he took the name Carnage. The symbiote gave Carnage super strength and he used it to break out of Ravencroft. Mordo then sent Carnage to Stark Enterprises to help Venom get the Inter-Dimensional Probe.
By working together Venom and Carnage were able to defeat Spider-Man and War Machine and they brought the Inter-Dimensional Probe to Baron Mordo. With the Inter-Dimensional Probe, Mordo could now open a portal to the Dark Dimension. However, Mordo told them that even with the portal open they needed to send an equal amount of life force into the Dark Dimension to replace Dormammu before he could come to Earth. Venom told Mordo that his debt was paid in full and left him. Carnage attempted to follow Venom but he was stopped by Dormammu. Dormammu then told Carnage that he didn't need Venom anymore and he would be enough to complete the next task.
Mordo gave Carnage the ability to grain people of their life forces. Carnage then went into New York City and began draining people of their life forces. Spider-Man attempted to stop Carnage but failed. After Carnage stole enough life forces he returned to Mordo's lair and Mordo had him drain the life forces into a special urn. However, Mordo told Carnage that to free Dormammu they still needed life forces from a few more people.
After Carnage absorbed the life forces out of a few more people Mordo teleported Carnage back to his lair.
Spider-Man knew that he needed to find Carnage so he and Iron Man went to Empire State University to ask Eddie Brock where Mordo was. Brock at this point had been separated from the symbiote. Brock told Spider-Man that he and Iron Man would stand a chance against Mordo, Carnage, and Dormammu and re-bonded to the symbiote to help Spider-Man and Iron Man defeat the three villains. Venom then took Spider-Man and Iron Man to Mordo's lair.
As Carnage returned he drained the last bit of life force into the urn and Mordo used the Inter-Dimensional Probe to open a portal to the Dark Dimension. At that moment Spider-Man, Venom, and Iron Man arrived and attacked them. Iron Man fought Baron Mordo and Iron Man was able to take him down. Mordo then ordered Carnage to release the life forces from the urn and send them into the portal. Carnage did as Mordo commanded and this freed Dormammu and he came through the portal and on to Earth. After a brief fight Spider-Man was able to send Dormammu back through the portal to his home world and they sealed the gateway. Mordo was able to escape and swore that eventually he would eventually free Dormammu from 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.