who is randall flagg?
It is a fictional character.
Randall Flagg
Randall Flagg is a fictional character created by American author Stephen King, who has appeared in at least nine of his novels. Described as "an accomplished sorcerer and a devoted servant of the Outer Dark," 
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Randall Flagg [SEP] Introduction
Randall Flagg is a fictional character created by American author Stephen King, who has appeared in at least nine of his novels. Described as "an accomplished sorcerer and a devoted servant of the Outer Dark," he has supernatural abilities involving necromancy, prophecy, and influence over animal and human behavior. His goals typically center on bringing down civilizations through destruction and conflict. He has a variety of names, usually with the initial letters "R. F." but with occasional exceptions, such as Walter o'Dim (originally envisioned by King as a separate character) in "The Dark Tower" series and Marten Broadcloak. Flagg first appeared in King's 1978 novel "The Stand" as a demonic figure who wreaks havoc after a plague kills most of the population.