Descent
Descent is a race league introduced in Season Three.
Areas
Descent areas differ from the usual areas or end-game maps. They are considerably larger, and some of them contain exclusive enemies that don't appear in the rest of the game. There is no towns in Descent, so the only sources of gear and crafting materials are random drops and chests located at the end of each area.
Once you enter an area, you can't go back. If you log out during the race, your character will respawn at the beginning of the current area. Similarly, there are no waypoints or any other quick means to travel between or skip areas aside from running through them. All areas contain bosses similar to their map counterparts, but they aren't necessarily placed near the exit.
Descent doesn't have the same "Normal—Cruel—Merciless" system where areas loop with increasing difficulty, but instead switches to Cruel and Merciless penalties starting with certain areas. For instance, Phrecia Outskirts is a Normal area, and the first Cruel area is Frozen Abyss.