The Leth is the deep interior of the continents, where the monsters are too strong for anyone to survive. Monster strength increases with distance from the settled coasts: the monsters nearest a settlement are the weakest of their kind, each day of travel inland brings stronger monsters, and deep enough in, no traveler survives. The depth at which the Leth becomes fatal depends on the travelers: a garrison dies at a depth a hero team can cross.

The Leth is on every continent, and roads and settled land keep to the safer ground at its edges. Sailors name the stretches of the Leth they pass, and settlements name the land behind their own fields; the same name applies to all of it.

Travelers enter the Leth only on purpose, and almost no one has reason to enter. Heroes go in after the monsters no army can reach. Other travelers who go deep seldom come back.