The Vale of Ruin is surrounded by "impassable" mountains (you can try jumping over the mountains). The game intends for you to go there only by moonbridge. The vale holds the unholy arc, the Draedoth Temple, and many evil wizards needed for quests. The Draedoth Temple holds the Shield of Retribution (a relic of power), and D.T. Khad.
The Vale of Ruin was the site of an ancient magical catastrophe. It is now sealed off, and can only be reached by the Moon Gates. The last known person who went in, Grymlock, was said to have been so horrified by what he saw that he lost his mind...
(2) Unholy Arc
(3) Altar of Fire
(4) Draedoth Temple
($) Treasure Chest