The second Dire Maul wing, the western one, is also known as the Capital Gardens, and contains a number of corrupted ancients and living and undead Highborne, a sect of elves that considered themselves the elite of the race. They separated themselves from the night elves, and one particular group, the Shen'Dralar withdrew to Dire Maul, cutting themselves off from the rest of the world.
Your first boss encounter in the zone is Tendris Warpwood, a corrupted Ancient of War. Defeating him gives you access to the inner portion of the gardens.
Your next objective is to disable the five pylons that are powering the prison of a powerful demon, known as Immol'Thar. The Highborne, led by the insane Prince Tortheldrin, are keeping the demon imprisoned and leeching his power.
Destroying the five pylons gives you access to the demon's prison, where you defeat him.
After although killing Immol'Thar is where the achievement for finishing the dungeon is granted, players can choose to continue onto Tortheldrin and end the corrupted prince's rule, if they so choose.