Shuma-Gorath is a fictional character created by Robert E. Howard, but better known for its later interpretations as a demon and Lord of Chaos appearing in the Marvel Universe. It is also well known for being a playable character in the Marvel Super Heroes video game, and made subsequent appearances in the Marvel vs. Capcom series.
Its first comic-book appearance was as an adversary for Doctor Strange in the pages of Marvel Premiere #3-9 (July 1972-July 1973) This re-imagining of the character resembled a giant purple eye with tentacles. In most subsequent appearances the color of the creature was changed from purple to green. Like Cthulhu, it was a giant tentacled creature which evoked terrible dread and was worshipped in time immemorial and long ago locked away from our plane of existence.
The character subsequently appears in Marvel Premiere #14 (March 1974), Doctor Strange #81 (February 1987), Strange Tales Vol. 2 #4-15 (April 1987-April 1988), Fantastic Four #314 (May 1988), Conan the Barbarian #252 (January 1992), #258-260 (July-September 1992), Fantastic Four Annual #27, Avengers #29 (June 1990), and Marvel Knights #26-27 (March-April 2006).
Shuma-Gorath received an entry in The Official Handbook of the Marvel Universe: Horror #1 (2005).
When Shuma-Gorath tries to return to Earth through the mind of the heroic sorcerer the Ancient One, he forces Doctor Strange to kill his master to prevent the powerful creature from arriving on Earth. Years later, Strange is forced to destroy the talismans keeping the chaos demons at bay, to keep them from falling into the hands of an evil sorcerer. Strange is then forced to travel to Shuma-Gorath's dimension and fight it there to prevent the demons from invading Earth. To do so, Strange absorbs the chaos magic into himself, gaining enough power to destroy Shuma-Gorath. However, he starts to transform into a new Shuma-Gorath himself as a result, so he commits suicide to prevent that from happening. Fortunately, an ally of Strange is able to purify him on his way back to the living. Shuma-Gorath resurrects himself not long after from the remnants of the chaos magic.
Shuma-Gorath is later summoned to Earth by the sorcerer Nicholas Scratch, who offers his services in return for "[his] life and Barbados". However, the demon is driven back by the combined efforts of the Fantastic Four, Doctor Strange, the Salem's Seven, and Diablo.
He also has the ability to generate several forms of energy attacks, mostly forming from his one eye. One attack not generated from his eye is when he forms a ball of pure energy that can shatter realities using all of his tentacles to generate the ball above himself.
He has powers similar to voodoo, which he demonstrated by creating a miniature replica of the planet Earth, and then transferring the effects on the replica to the real earth. He is capable of draining the magical energy of other beings, as Sise-Neg noted.
He can also teleport and phase at will and levitate. He can enter and leave different planes of existence and dimensions at will and has reality warping powers. He can also summon demons and other supernatural creatures to do his bidding.
His intelligence is nearly immeasurable, approaching omniscience.
In his video game appearances he demonstrates various magical abilities, including energy projection, shape-changing, teleportation, and vampiric abilities, as well as the ability to project explosive eyes as projectiles.