The Black Morass is a dungeon inside Caverns of Time which is at the mideastern corner of Tanaris. You need to be level 66 to enter and the dungeon consists of level 66-73 elite mobs.

Guide for Dungeon: The Black Morass
Guide for Dungeon: The Black Morass

The dungeon consists of 3 bosses; Chrono Lord Deja, Temporus and Aeonus. There is a quest to complete the dungeon event and can be obtained inside the dungeon;

Quest: The Opening Of The Dark Portal

When you enter the instance you need to run directly south to find Medivh. Wait near him and you will see a portal spawn at a random location. Along with the portal a Rift Lord or Rift Keeper will spawn. Go to it and kill the lord or the keeper to close that portal. You should also kill all the other non-elite mobs coming out before they can reach Medivh. The non-elites consist of single strong mob or a group of 3 weak mobs. You will need to stop 18 portals for this event to end. From the 6th, 12th and 18th portals, instead of a lord or a keeper a boss will come out (Chrono Lord Deja, Temporus and Aeonus in order). The portals are on a timer and if you lose too much time with one the other might spawn without you finishing with the first one. After the boss battles there will be a little pause before the following portal to spawn for you to rest, other than that you will not find the time to rest. If a mob reaches Medivh he will not instantly die of course. Instead he will start to lose his shield. Medivh’s shield start from 100% and decreases with each attack. You can check the shield’s status with the screen text.

By killing Aeonus the first time, you can also receive Achievement: Opening of the Dark Portal.