Dark Portal

The Dark Portal, also known as the Great Portal, is the gateway between Azeroth and Outland. The Azeroth side of the Portal is located in the Southeastern reaches of the Blasted Lands of Southern Azeroth, while the Outland side, the Stair of Destiny, is located in the Eastern portion of Hellfire Peninsula.

History
The Dark Portal was created by a joint effort between Medivh, possessed by the Dark Titan Sargeras, and the warlocks of the Shadow Council, which allowed the Horde to invade the world of Azeroth. The Portal bridged the distance between the worlds of Azeroth and Draenor, and it was large enough that individuals might pass through it. Gul'dan immediately dispatched orc scouts through the Portal in preparation for the coming invasion. After the Portal was widened, the armies of the orcs poured through it into the unsuspecting Kingdom of Azeroth, beginning the First War.

Following the defeat of the Horde in the Second War, the Dark Portal was captured by the forces of the Alliance of Lordaeron. Hoping to end the threat of the orcs, the Archmage Khadgar attempted to close the gateway to Draenor. While the structure was damaged/destroyed, the link between the worlds remained as a tear in the fabric of reality. To keep watch over the remnants of the Portal, Nethergarde Keep was constructed.

Using a magical cloak of darkness to escape the detection of the mages at Nethergarde, the Bleeding Hollow clan used the rift to return to Draenor. Hearing the Bleeding Hollow's tales of Azeroth, the Elder Shaman Ner'zhul, Chieftain of the Shadowmoon Clan and Warchief of Draenor, planned to open a number of portals on Draenor that would lead the Horde to new worlds to conquer. To do so, Ner'zhul required mystical artifacts - the Book of Medivh, the Skull of Gul'dan, the Jeweled Scepter of Sargeras, and the Eye of Dalaran. In order to procure these artifacts, the elder shaman reopened the Dark Portal and sent his forces through. King Terenas of Lordaeron, convinced that the orcs were preparing a new invasion of Azeroth, assembled his most trusted lieutenants, including Khadgar and the paladin Turalyon, to form an expedition through the Portal to put an end to the orcish threat once and for all. Ner'zhul finally opened his portals to other worlds, but he did not foresee the terrible price he would pay. The portals' tremendous energies tore Draenor apart. Grom Hellscream and Kilrogg Deadeye - who saw that Ner'zhul's portals would destroy their world, and his insanity would destroy their people - led their clans through the Dark Portal back to Azeroth. The Alliance forces remained on Draenor, agreeing to make the ultimate sacrifice by destroying the Dark Portal from their side in order to prevent that world's destruction from destroying Azeroth as well. There were indications they may have taken one of Ner'zhul's portals to another world, but they managed to survive Draenor's destruction, and remain in the shattered realm now called Outland to this day.

The Dark Portal remained dormant for twenty years, until an artifact of incredible power was found by the demon Lord Kazzak, a lieutenant of Kil'jaeden. Kazzak used the powerful artifact to reopen the Dark Portal and cross into Outland.