Described as "an accomplished sorcerer and a devoted servant of the Outer Dark",[1] he has supernatural abilities involving necromancy, prophecy, and influence over animal and human behavior.

Randall Flagg makes his first named appearance in King's 1978 apocalyptic novel The Stand, where he tries to construct a new civilization in the United States after a plague kills most of the population.

Flagg's background is vague, even to him; he says that at some point he just "became", although he remembers being a Marine, a Klansman, and a Viet Cong member, and having a hand in the kidnapping of Patty Hearst.

Flagg plans to attack and destroy a rival emerging civilization—Mother Abagail's Free Zone in Boulder, Colorado—to become the dominant society in the former United States.

[3] After two of Flagg's followers fail to kill the leaders of the Free Zone, the Boulder community sends a group of men to Las Vegas to stop him.

As the other two are being prepared for a public execution, one of Flagg's most loyal followers, the Trashcan Man, arrives with a nuclear warhead salvaged from a military base.

[5] In this novel, Flagg schemes to throw the kingdom of Delain into chaos by poisoning the king and framing Prince Peter, the legitimate heir to the throne, for the crime.

In flashbacks Flagg assumes the identity of Marten Broadcloak, a wizard who conspires with the Crimson King to cause the fall of the Dark Tower.

In the city of Lud, Flagg saves Tick-Tock Man Andrew Quick, an enemy of Roland's ka-tet left for dead in an earlier confrontation.

In flashbacks, the reader learns that Flagg as Walter o'Dim was an emissary for John Farson, one of those responsible for the destruction of Roland's home Gilead.

[1] The "Argument", a summary of the series thus far, beginning Wolves of the Calla—the fifth novel in the series—notes that Flagg is known as Broadcloak, Fannin and John Farson, depending on the world in which he lives.

[17] His appearance in Song of Susannah is via a flashback revealing that Flagg bargained with the succubus Mia; this resulted in the birth of Mordred Deschain, son of both Roland and the Crimson King.

[18] The Dark Tower reveals more of Flagg's background, relating that he was born Walter Padick in Delain to Sam the Miller of Eastar'd Barony.

[21] In Hearts in Atlantis (1999), Raymond Fiegler is identified near the novel's end as leader of an activist group when he prevents Carol Gerber from retrieving an unexploded bomb on a college campus.

Director Mick Garris and the studios wanted to give the role to an established star such as Christopher Walken, James Woods, Willem Dafoe or Jeff Goldblum.

[6] King's idea for the role was someone who "would make the ladies' hearts go pitty pat, that looked like the type of guy you would see on the cover of one of those sweet, savage love paperback romances".

[36] In April 2009, Marvel released a single-issue comic written by Robin Furth and illustrated by Richard Isanove entitled The Dark Tower: Sorcerer, which focused on the character of Marten Broadcloak-Walter o'Dim.

[37] Sorcerer provides an origin for the character that is different from the one King initially wrote, explaining that Walter was the son of the wizard Maerlyn and Selena, Goddess of the Black Moon.

Furth introduced the idea that the Bends o' the Rainbow, 13 magic spheres created by Maerlyn in the distant past, are sentient beings able to project personifications which can interact with other characters.

This is described as incestuous, since the beings were given life by Maerlyn, Walter's biological father; Marten and the Grapefruit repeatedly call each other as brother and sister.

He is quite a demonic figure, and as such he is one of the great anti-heroes of contemporary popular fiction" and that "journeying into Walter's mind is a pretty wild experience and at times a little frightening.

"[41] To find Walter's voice, Furth went to John Milton's Paradise Lost, William Blake's Proverbs of Hell, the Biblical Song of Solomon and the writings of Aleister Crowley for inspiration.

[42] In his interview with Bev Vincent, Isanove opined that Walter was his favorite character to draw; "Jae [Lee, the original artist for the series] established him as almost androgynous.

Flagg is like the archetype of everything that I know about real evil, going back all the way to Charles Starkweather in the '50s—he is somebody who is empty and who has to be filled with other people's hates, fears, resentments, laughs.

To her, the combination of these two characteristics found in different cultural realms forces people to face their "flawed humanity" with the "amorality" Flagg represents.

[53] Douglas Winter, author of Fear Itself: The Horror Fiction of Stephen King, believes that Flagg epitomizes the Gothic villain—an "atavistic embodiment of evil"—since his appearance is indistinct, malleable and a "collection of masks".

According to de Camp, absolute evil is hard to envision; whereas Adolf Hitler and Joseph Stalin believed they were improving the world, Flagg only enjoys causing destruction and chaos.

Here, Magistrale states that in The Stand Flagg gives the reader an "illustration of King's jaundiced perspective of modern America" as he presents the consequences of technology—worship and the sacrifice of "moral integrity to the quest for synthetic productivity".

Patrick McAleer, author of Inside the Dark Tower Series: Art, Evil and Intertextuality in the Stephen King Novels, argues that Flagg's situation is the most sympathetic of all of King's characters, and his evil may be retribution: "[I]n suspending any disbelief in the possibility that reprisal is a reaction to rape, the life of Flagg becomes one that looks to strike a balance for the sexual crime committed against him.

McAleer compares Flagg to Satan in Paradise Lost, suggesting that he may be another "fallen angel who has a valid case supporting his devilry".