From what I know, he's a sorceror from the far future who somehow gained godlike power and took Doctor Strange and Baron Mordo back through time and created a universe (just one universe, doesn't put him on TOAA level)
From the Brunner run on Dr. Strange in the 70s. Sise-Neg was sorceror from the future, where there was little magic left, who traveled backwards through time, absorbing magical power, including that of Mordo, Strange, Merlin, and Shuma-Gorath (who he defeated like a gnat) until he had arrived at the beginning of the universe, and had absorbed not only all the magical energies in the universe, but all the rest of the power as well. When arriving at the beginning of time, Doc Strange finally prevails upon him to utilize his powers for good, and in doing so, he creates the big bang that started the 616 universe, renaming himself Genesis.
It's hard to tell, really. A lot of those early stories seemed to take place out of the true Marvel Universe. I mean, things happened to the entire earth/universe/multiverse that's never mentioned in other comics.
it's one of those kinda situations where you just have to accept that they're just stories written to entertain- not every writer is going to follow exactly previously continuity, and they shouldn't have to.
Or you could rationalize it by saying that Sise - Neg went back in time and remade the universe, and that undid all the things that he had done, erasing them from the timestream so that only Doctor Strange and Baron Mordo remembered them.