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.
