Baron Karl Amadeus Mordo (known as Baron Mordo) is a character appearing in American comic books published by Marvel Comics, commonly as an adversary of Doctor Strange. The character was created by writer Stan Lee and artist Steve Ditko, and first appeared in Strange Tales #111 (August 1963). Baron Mordo is a gifted magician, especially adept in the black arts of magic, including summoning demons.

Karl Mordo was studying the magic arts under the Ancient One in Tibet when Dr. Stephen Strange arrived. Strange foiled Mordo's plot to kill the Ancient One, leading to Mordo being cast out and Strange eventually becoming Sorcerer Supreme. Mordo has since clashed several times with Dr. Strange, at times with the backing of the demon Dormammu, briefly even impersonating Strange.

The Baron Mordo character has appeared in other forms of media, such as animated television series, films, and video games. Chiwetel Ejiofor portrays the character in the Marvel Cinematic Universe film Doctor Strange (2016) and an alternate universe version in Doctor Strange in the Multiverse of Madness (2022).

Baron Mordo first appeared in Strange Tales #111 (August 1963), and was created by Stan Lee and Steve Ditko.

A Transylvanian nobleman, Karl Mordo became a student of the Tibetan sorcerer known as the Ancient One. When Mordo plotted to kill his teacher, Dr. Stephen Strange learned of the plot. Mordo was forced to cast restraining spells to prevent Strange from warning the Ancient One. In desperation, Strange decided to accept the Ancient One's offer to take him on as his mentor in magic to have some hope of stopping Mordo. The Ancient One, however, was fully aware of Mordo's plot and of Strange's desire to warn him. Pleased at Strange's unselfish decision, the Ancient One's magical teachings immediately freed the doctor and explained the full situation. Thus informed in the face of his recent experiences, Strange agreed to become the Ancient One's apprentice despite the requirement that he abandon his former life. The Ancient One trained him to be a formidable opponent of Mordo. Eventually Mordo was exiled by the Ancient One. Mordo's abilities were similar to those of Doctor Strange, but Mordo was particularly skilled at astral projection and hypnosis, as well as mesmerism. He was more than willing to use powerful black magic and invoke demons, both of which Strange was reluctant or unable to do. Mordo's use of these darker arts would sometimes backfire.

The evil Mordo became an open foe of Doctor Strange. He sent his astral form to hypnotize the Ancient One's servant into poisoning his food, hoping the old man would reveal his secrets of magic. However, Strange tricked him into returning to his physical body after engaging in astral contact with him, thus breaking his hold over the servant, who helped the Ancient One to recover. Mordo disguised himself as Sir Clive Bentley, and trapped Strange with a drugged candle that paralyzed him, but Strange was able to use his mental powers to call a local girl to free him. He opposed Strange's discipleship to the Ancient One.

Mordo makes a deal with his new master, Dormammu, to amass additional power to defeat Doctor Strange. Mordo sells his soul to both Mephisto and Satannish for power, gaming that Strange would save him.

Mordo contracts terminal cancer as a side effect of his use of black magic, and renounces evil just before his death. He later returned to life. Mordo allies with Terrax, Tiger Shark and Red Hulk as the "Offenders", to be opposed by Doctor Strange and the three original Defenders.