Undoubtedly the most mysterious character within the Marvel Multiverse. The One-Above-All has only appeared and been mentioned a few times. He is not to be confused with One Above All, the leader of the Celestials, or any religious figures. The One-Above-All was first mentioned by the Living Tribunal when Doctor Strange first encountered the entity. The-One-Above-All is often unofficially cited as Marvel's creator.
When his final confrontation with Asteroth nearly left him dead, Beta Ray Bill was restored to life by a mysterious, glowing white-robed man (who was clearly one of the more anonymous representations of The One-Above-All.)
When the Fantastic Four journeyed far beyond the boundaries of existence they ended up in The One-Above-All's personal realm where he took the form of Jack Kirby. After a short conversation with him The One-Above-All healed Reed's severely damaged face and sent them all home.
The One-Above-All's was seen taking the guise of a homeless man and was advising Peter Parker (aka Spider-Man) to be with his Aunt May who was dying at the time as well as telling Peter that death is a natural part of life and we as people must learn to let those we love who are dying go when the time has come. Peter did not listen to what The One-Above-All said.
The One-Above-All's first appearance is considered to be Doctor Strange #13, written by Steve Englehart with art by Gene Colan. The character can often present problems to readers in the sense that meta layers can be interjected with the character on various occasions, subtle references to past writers and creators, and its often very ambiguous and vague nature. Mark Waid's Fantastic Four run had an appearance of the character, and he cites Tom Peyer as the person who suggested The One-Above-All could be represented by a comic artist, specifically Jack Kirby. Writer and artist of the issue didn't want to be too specific though as to not put off casual readers or be too "in-jokey" and they managed to sneak in a cigar in an ashtray circumventing Marvel's policy of having no characters smoking but also recognize Kirby who was often told to have a cigar on him.
The One-Above-All is Omnipotent (All Powerful), Omniscient (All Knowing), and Omnipresent (Everywhere). He is the most powerful being in the Marvel Multiverse. He exists beyond space and time. His power is unlimited, incalculable and immeasurable, he cannot be damaged in any way and he has control over all forms of energy.