It's a fantasy role-playing game.

DM: You are Keyleth, a half-elf druid . You are the member of the famous group of adventurers. Previously, The battle still rages across the square, and as the party looks back the way they came they see a large group of figures through the rain. As the sun sets and the figures approach, it becomes clear that it is a group of thirty to fifty armored skeletons marching in from the north.
DM: Okay. (foghorn sound) The slow horn-blow comes out, and this fog begins to fill the area between this marching skeletal army and the Sun Tree where you guys currently are. Looking over your shoulder, you can see the zombie giant finishes smashing the last individual in that crew, picks it up and jams it into its jagged giant teeth.
Player: So there's a big army coming down, filtering down a street.
DM: Down the street towards you.
Player: How far away?
DM: You can't tell now. There's just fog along the way.
Player: I can't tell how far away they are right now?
DM: No, you can't see them.
Player: I can't see them?
DM: No, because Scanlan just created a whole bunch of fog between the two of you. The moment you saw, they may be about a hundred or so feet from you.
