1 Alvissa: Hurry up. Night falls quickly in these parts.
2 {it's before dark}
3 {it's dark}
4 Mordekai: Yeah, I'll give you that.
4 Gollum: Gollum!

Twilight is something everyone has some familiarity with. It's basically the time between the sun rising or setting and the sky being dark. The technical definition is more complicated, but you can think of it that way.

Many people reading this live in parts of the world where there is significant twilight. There may be an hour or more of time after sunset when it's still light enough to read by the remaining glow in the sky and before it gets completely dark.

This time is however dependent on your latitude. The closer to the north or south pole you are, the shallower the angle of the sun's path through the sky, which means that when it sets, it takes longer to move vertically below the horizon, as much of its motion is horizontal. In the tropics, the path of the sun is much more vertical, so when it hits the horizon it drops below it much more quickly. Twilight in the tropics can be very short indeed - a matter of a few minutes.

Living at a relatively low latitude compared to most people in the developed world, I'm used to a relatively short amount of twilight. Basically, once the sun sets, you know that the stars will be out and it'll be totally dark within 20 minutes or so. When travelling to higher latitude locations, I really notice the extra length of the twilight.

Evidently the Swamp of Terror is somewhere near the equator. (Or it's just magic.)