Flagg is the Man In Black from Dark Tower.
In Roland's world, the World Moved On, there are "thinnies" to alternate versions of our world, or what version of it Mr King is working in at that time. The Stand was a very early book, as was The Gunslinger, and Eyes Of The Dragon, the other book in which Flagg appears.
If you look at his other early works, Carrie, Salem's Lot, Shining, Dead Zone, the four stories in Different Seasons which include Shawshank and The Body (Stand By Me), he hadn't really started making a lot of those connections that you see in his later works. Roland's world really was a different story set in a different world at that time. The only connections at that time were that a lot of stories were set in Maine, and more of them were being set in, or connected to, Castle Rock.
The Stand was the first early book where the character leave the East Coast and march west, and it is a post apocalyptic brave new world. The next tie he tried this post apocalypse approach was in Cell, the first post-Roland book, written after completing the last DT.
