The Haunted Quarter is where the evil djinni, Ezrek Khan, and his Netherim followers reside. It is a sort of "nether-realm" that serves as the backdrop for Fazeer Shah's final task - the terrain and architecture is much like the Salt Barrens, but the skies are magenta and stormy, and large tentacles grow from the depths and sway overhead...
It is infested with the Netherim, with whom Khan has allied himself. Beware the fast-moving tentacles, which will attempt to smash you if you come too close!
Jehannum is the small sub-zone of the Haunted Quarter where Ezrek Khan resides, where the skies are more red than purple, but the architecture and scenery remain much the same (without the smashing tentacles). The entrance to the zone is separated from the arena where the boss fight occurs by a teleport pad, much like in many other dungeons such as the Forsaken Vaults.