5e DMG4:
PART 1 : MASTER OF WORLDS
Every OM is the creator of his or her own campaign
world. Whether you invent a world, adapt a world from
a favorite movie or novel, or use a published setting for
the D&D game, you make that world your own over the
course of a campaign.
The world where you set your campaign is one of
countless worlds that make up the D&D multiverse,
a vast array of planes and worlds where adventures
happen. Even if you're using an established world such
as the Forgotten Realms, your campaign takes place
in a sort of mirror universe of the official setting where
Forgotten Realms novels, game products, and digital
games are assumed to take place. The world is yours to
change as you see fit and yours to modify as you explore
the consequences of the players' actions.
Your world is more than just a backdrop for
adventures. Like Middle Earth, Westerns, and countless
other fantasy worlds out there, it's a place to which you
can escape and witness fantastic stories unfold. A well
designed and well-run world seems to flow around the
adventurers, so that they feel part of something, instead
of apart from it.
Consistency is a key to a believable fictional
world. When the adventurers go back into town for
supplies, they should encounter the same nonplayer
characters (NPCs) they met before. Soon, they'll learn
the barkeep's name, and he or she will remember
theirs as well. Once you have achieved this degree of
consistency, you can provide an occasional change. If
the adventurers come back to buy more horses at the
stables, they might discover that the man who ran the
place went back home to the large city over the hills,
and now his niece runs the family business. That sort of
change-one that has nothing to do with the adventurers
directly, but one that they'll notice-makes the players
feel as though their characters are part of a living world
that changes and grows along with them.