He is defeated (with the help of some divine power) when the Trashcan Man brings an nuclear weapon into Las Vegas, which explodes. However, we know that Flagg does not die in this blast, as he appears in the epilogue ona beach, with a tribe of primitive men.

Is Randall Flagg Pennywise?

Randall Flagg is an “accomplished sorcerer” whose powers of dark magic make him high-on immortal, where Pennywise is an ancient evil deity. ... Not only do the two both share the list of similarities outlined above, but Flagg and Pennywise are also both recurring villains who appear in numerous King novels.

How is Randall Flagg defeated in the stand?

At the end of The Stand (spoiler alert), Flagg is defeated when a literal, giant glowing hand — the Hand of God — intervenes and destroys his followers, though Flagg himself is revealed to have survived in an epilogue in The Complete and Uncut Edition of the novel.