Who or what exactly is Shuma Gorath?
Well as with all demons its very hard to pin-point the entire details about a particular demon, specially an extradimensional one, however, based on what i have collected of Shuma Gorath, here is what i have drawn of what Shuma Gorath actually is.
We know Shuma Gorath is a class 3 demon (Marvel Handbook 2005)
Demons in marvel univese is classified into 3 types, not based on their power level but rather based on their origin and appearence. A class 3 demon is one that was born in a dimension other than that of the earth (extradimensional) and does not have a humanide appearence. This is explained below, from a very old Marvel Handbook 1983
The same thing is again said on Marvel Handbook 1986, just putting this as its easier to read
Marvel Glossary also gives us a definition of what extra-dimensional actually means:
Shuma Gorath's bio in Marvel Handbook 2005 helps up fill in more information
Shuma Gorath is likely impossible to destroy permanently (Dr. Strange had achieved something a lot closer)
Virtually omnipotent in his own realm
Can draw energies from other be it mystical or otherwise
It has been said that he is one of the primal power of Chaos, and Atlantis sorcerer are beyond inadequate to even deal with problem concerning Shuma Gorath (Invaders Now 04)
It also gives us a power grid, bonus ;)
Kalu believes Mephisto and Sattanis are nothing compared to Shuma Gorath, but this is said in regards to when Dr. Strange was about to meet Shuma Gorath in his own domain, so i take this as in his domain Shuma Gorath >> Mephisto and Sattanis (Strange Tales, Vol 2, 13)
Now lets begin with his publication history
Marvel Premier 09: Shuma Gorath sitting in his own dimension takes control of Ancient One's mind on earth.
Marvel Premier 10: Dr. Strange shrinks himself, enters AO mind, fights Shuma Gorath, is unable to beat him and his forced to kill Ancient One in order to prevent Shuma Gorath from entering Earth.
Phew! thats well pretty much the entire issue there, took me ages to load.
Marvel handbook 2005 vouches the entire incident
We also see a reference to this incident in Invaders Now 04
Marvel Premier 14: Shuma Gorath was defeated and banished by Sies Neg, but this honestly doesnt tell us a lot given Sies-Neg was pretty close to ascending to God Hood and after banishing Shuma Gorath he goes on to re-create the whole universe.
Marvel Handbook 2005 verifies the whole incident
There is a subtle reference to his loss in the hands of Sise Neg in Invaders Now 04, where one few items that could help purge Shuma Gorath influence on earth was a flower name Sise Neg.
Strange Tales Volume 2, 13: Dr. Strange is trying to enter Shuma Gorath's realm in order to stop him, he is force to fight Arioch one of Shuma Gorath's minion, and in the fight ends up absorbing Arioch. This i put in because this is important on how Dr. Strange did the near impossible task of beating Shuma Gorath in his own realm.
Strange Tales volume 2, 14: Here Dr. Strange does the near impossible, beats Shuma Gorath in his own realm. This is however done using plot, first by being force to use black magic against Arioch and having absorbed him, this help Dr. Strange transform to something closer to shuma gorath, leading him to actually absorb Shuma Gorath's essence and finally beat him. In essence, because of plot, Shuma Gorath ended up fighting his very own powers, and that is what destroyed him
Since this is a little difficult idea lets spend a bit more time trying to understand what happened and of course i'll follow up with marvel bio to substantiate it.
Firstly, when Dr. Strange meets Shuma Gorath he states how Dr. Strange has changed because of the use of Black Magic.
A little philosophy "He who fights too long with Dragons becomes a Dragon himself"
So the use of black magic changed him and Shuma Gorath notes if you fight long enough with the dragon you become dragon himself, i take this to mean you fight with Shuma Gorath long enough you become Gorath himself.
This panel depicts more change that has occurred in Dr. Strange, he is becoming more and more aggressive, understandable as he is fighting a primal god of chaos and tapping into similar energy
More reference straight from Shuma Gorath that he has changed.
Then Shuma Gorath finally comments on how he was absorbing his own powers
Marvel handbook 2005 verifies it, and it also points that it was the use of black magic that led him to become Shuma Gorath
Doctor Strange volume 2, 15: Dr. Strange now has become Shuma Gorath, and is so powerful than his mere existence would destroy Galaxies (an unsubstantiated claim apart from the statement), he then slowly gives away his power turning back to Dr. Strange
Fantastic Four 314: The fight between Shuma Gorath and Dr. Strange had the potential to destroy ALL of neither realms.
Conan the Barbarian, volume 1, 260: Conan beat Shuma Gorath using Iron Bound Books
Despite the weird appearance this is canon to Shuma Gorath's chronology, and is substantiated by Marvel Handbook 2005
Sorry i dont have bios after this point, if someone can find supporting scans from bio please feel free to add them
Marvel Knights 4, Issue 26: Nicolas Scratch tricks Fantastic Four into releasing Shuma Gorath from his prison, as the spell has to be read by someone with pure heart and presumably nice rack ;)
Marvel Knights 4, Issue 27: We see a new weakness so Shuma Gorath, vertigo. We also see FF helpless against Gorath, but apart from Johnny the rest were depowered by drinking a tea that depowers them.
Enters Dr. Strange who knowing he cant beat him in presence of Chaos, teleports him in the stone henge and beats him there, the fight sadly takes place off panel.
Reference to this was placed during Invaders 04
Wolverine First Class, Issue 12: Quoggoth, one of the creations of Shuma Gorath, rebels against Shuma, is defeated by Shuma who fails to kill him so Shuma imprisons him here on Earth.
Invaders Now, Issue 04: Shuma Gorath's powers were invoked to bring dead people back to life, and the spell would have succeeded had the dead not refused to come back.
Invaders Now 05: Vision poked its eye, then used the spear of destiny to close the portal, thereby preventing Shuma Gorath from entering earth.
He was next mentioned during Thanos Imperative, I am a little hesitant to use those as Shuma Gorath himself never cleared appeared on panel, but if i get request to put those up i shall.