The Dark Portal is the gateway between Azeroth and Draenor located within the Blasted Lands that facilitated the first conflicts between orcs and humans. Orginally created by a joint effort between Medivh, possessed by the Dark Titan Sargeras, and the warlocks of the Shadow Council, the portal allowed the Horde to invade the world of Azeroth.
Following the Horde's defeat in the Second War, the Dark Portal was captured by forces of the Alliance of Lordaeron, who attempted to close the portal. However, they only succeeded in damaging the structure, the link between the worlds remaining as a tear in fabric. To keep watch over the remains of the portal, the fortress of Nethergarde Keep was built.
Nethergarde's defenders weren't infallible, however, and the Bleeding Hollow Clan successfully evaded detection as they used the rift to return to Draenor. Hearing their tales, the Elder Shaman Ner'zhul planned to open a number of portals to other dimensions to lead the Horde to new worlds. To do so, he needed four artifacts of power: the Book of Medivh, Skull of Gul'dan, the Jeweled Scepter of Sargeras and the Eye of Dalaran. Ner'zhul reopened the Dark Portal and sent his forces through to retrieve these artifacts.
King Terenas Menethil II of Lordaeron, convinced that the Horde was preparing another invasion of Azeroth, assembled his most trusted lieutenants, including General Turalyon and Archmage Khadgar, to lead an expedition through the Dark Portal and put an end to the orcish threat once and for all. However, they were unable to prevent Ner'zhul from opening his portals to other worlds.
Ner'zhul finally opened his portals to other worlds, but he did not foresee the terrible price he would pay. The portals' tremendous energies began to tear the very fabric of Draenor apart. As Turalyon's forces fought desperately to return home to Azeroth, the world of Draenor began to buckle in upon itself. Grom Hellscream and Kilrogg Deadeye, realizing that Ner'zhul's mad plans would doom their entire race, rallied the remaining orcs and escaped back to the relative safety of Azeroth.
On Draenor, Turalyon and Khadgar agreed to make the ultimate sacrifice by destroying the Dark Portal from their side. Though it would cost their lives, and the lives of their companions, they knew that it was the only way to ensure Azeroth's survival. Even as Hellscream and Deadeye hacked their way through the human ranks in a desperate bid for freedom, the Dark Portal exploded behind them. For them, and the remaining orcs on Azeroth, there would be no going back.
The Dark Portal remained dormant for twenty years, until the demon Lord Kazzak found an artifact of great power that allowed him to reopen the portal. The portal turned from blue to green, showing that it is active. After crossing back into Outland, a combined force of the Alliance and the Horde was set up in the portal's crater.
The deposed Warchief of the Horde, Garrosh Hellscream, linked the Dark Portal of Azeroth to that of another portal built in an alternate time way in which the orcs never drank the blood of Mannoroth. With most of the clans under him under the banner of the Iron Horde, they invaded Azeroth, seizing the lands surrounding the portal, capturing Dreadmaul Hold and destroying Nethergarde Keep.