All Mount Hyjal is an instance, making it the biggest outdoor instanced zone in World of Warcraft. The entrance is at the southwestern corner of the Darkwhisper Gorge— within southern Winterspring region. The instance gate is closed by a wooden-door. You can see through the door, that the zone-in whirlpool is green, meaning 40-players raid. Here is where the epic battle of the forces led by Thrall , Jaina Proudmoore and Malfurion—united by Medivh’s spirit—defeated Archimonde and the Burning Legion. Expect it to be an intense high-end content. The Darkwhisper Gorge is guarded by Elite Felguards, Succubis and Felhunters, so it is obvious Mount Hyjal has a strong Burning Legion presence.
Mount Hyjal will be available in the Burning Crusade Expansion. Through Caverns of Time we will timetravel to the past and witness first hand the War of Hyjal that took place during Warcraft III: Reign of Chaos game.
However, players will be able to enter Mount Hyjal in the present time as well. The instance has a a sub-instance within. In the outside, you will see a replica of Onyxia’s lair—which represents a possible Dragon encounter. There is a huge blackened crater near the top of Mount Hyjal, similar to the crater of the Dark Portal. What will be placed there is a mystery. At the top of Mount Hyjal is a big lake—representing the new Well created by the 3 vials poured in by Illidan after the Sundering. Above the lake is a grown Nordrassil World Tree.
Back in Warcraft III: Frozen Throne Game, Malfurion said that the World Tree’s roots have been healing well (Read more).
The World of Warcraft RPG: Lands of Mystery page 17—released on April 3, 2006—has details of Mount Hyjal. Prospector Brann Bronzebeard adventured into Mount Hyjal and claims that Demon Hunters, Keepers of the Grove and Dryads are assisting the Red, Blue and Green Dragonflights in guarding the World Tree. A large number of demons inhabit the surrounding areas of Mount Hyjal, specially at the Darkwhisper Gorge. Brann speculates that the demons might be digging underneath Mount Hyjal, attempting to tap directly into the second Well of Eternity.
There is no official info from the Developers of World of Warcraft concerning Mount Hyjal—the present version. However, there will be a past version available through the Caverns of Time. Two Mount Hyjal instances?—you wonder.
So, there is obviously two copies of the instance: the present version and the past version.