Who Is Shuma-Gorath In The Marvel Universe?
Deities have always been an integral part of the Marvel Universe. From its earliest days, gods such as Thor and Odin have been central to the development of Marvel's mythology, despite the fact that, at face value, such entities would seem to conflict with the more down-to-earth or sci-fi heroes. After all, how can the Norse God of Thunder exist in the same universe as a down-to-earth character like Clint Barton, or a star-trekking space hero like Captain Marvel?
Somehow, Marvel managed to make this mishmash of styles, genres, and inspirations make sense together. This remains true even as you dig past the more popular heroes into more obscure characters — because Marvel has done much more than fold existing mythology into its canon. In fact, it has folded other fictional universes in, as well, whilst somehow maintaining its originality and identity. Perhaps no character is more indicative of this than the monstrous being known as Shuma-Gorath.
Like Thor, Shuma-Gorath is a deity. Unlike Thor, though, it is an original character. However, even Shuma-Gorath has its inspirations, the most notable being the works of H.P. Lovecraft and Robert E. Howard (i.e. the creator of Conan the Barbarian and the Hyborian Age). To date, Shuma-Gorath hasn't yet appeared in the MCU (though there was at point speculation about a "WandaVision" appearance), so the entity remains largely unknown with the general public — a fact which belies its overwhelming level of power, and just what an incredible menace it can be.
Shuma-Gorath is Marvel's ode to eldritch horror
Fans of Lovecraftian horror are probably familiar with many of the disturbing concepts surrounding Shuma-Gorath. Originally created in 1972 by Marvel writers Steve Englehart and Frank Brunner, Shuma-Gorath — a name which the two took straight from Howard's writings, in fact — is clearly Marvel's way of bringing that sort of eldritch horror into its universe (via Comics Explained).
Depicted as an immortal, timeless eyeball surrounded by green tentacle appendages, Shuma-Gorath is what is known as a Great Old One. As a Great Old One (or a Many-Angled One), Shuma-Gorath's entire purpose revolves around traveling between realities, conquering them, and forcing its inhabitants to worship it. This deity predates the universe itself, and has conquered millions of universes across its lifespan. It is so powerful that even big extra-dimensional baddies like Mephisto are considered small potatoes when compared to Shuma-Gorath, and when it does make rare appearances within superhero comic books, it's an extremely big deal.
As you might expect, a being of Shuma-Gorath's stature is incredibly powerful. It possesses the ability to travel between and manipulate realities, destroy galaxies, and probably a whole host of other things that readers have not seen. In the few times Shuma-Gorath has posed a direct threat to Earth, Marvel's heroes have only ever been able to banish it: nobody has ever quite "defeated" Shuma, if such a feat is even possible.
Only powerful sorcerers are capable of warding off Shuma-Gorath
Shuma-Gorath is far too big an entity to only be connected to one major Marvel superhero. As a primordial deity of pure evil, Shuma is essentially impossible to kill, and even banishing it requires the ability to wield powerful magic. That's why, typically, the characters who most often confront Shuma-Gorath tend to be magic wielders like Dr. Strange and the Scarlet Witch, with the former being its most frequent Earth-based opponent — and even he will often find himself at wits' end when he opposes the deity.
In their first encounter in "Marvel Premiere" Vol. 1 #10, Strange was forced to kill his mentor, the Ancient One, in order to prevent Shuma-Gorath from traveling to earth through the Ancient One's mind. Another encounter in "Strange Tales" Vol. 2 #14 saw Strange merging himself with Shuma-Gorath in order to use its own power against it and prevent another invasion. During the Hyborian Age, Conan of Cimmeria assisted in sealing away Shuma-Gorath through the use of the Iron-Bound Books (as depicted in "Conan the Barbarian" Vol. 1 #260). A fairly recent appearance, in "Mighty Avengers" Vol 2, #3, saw Shuma-Gorath materializing in Times Square, and facing off against the Avengers.
Despite the being's overwhelming power and influence, Shuma-Gorath has only ever appeared a handful of times throughout Marvel's history. Much of the character's mainstream exposure comes from its appearance in fighting games like "Marvel Vs. Capcom 3."
Even so, the Shuma-Gorath remains one of Marvel's most interesting antagonists, and a fine example of how Marvel's writers have been able to integrate a variety of influences into one universe. Now, will this monstrous being ever appear in the MCU? If it did, then "Doctor Strange in the Multiverse of Madness" would be the most likely place, so keep an eye out.