The Dark Portal is the symbol of the Warcraft universe. It was the loading screen for the original game, for the Burning Crusade expansion, and it will be again for the new Warlords of Draenor expansion as well. When we think of the orcish Horde pouring into Azeroth to murder and plunder, it's the Dark Portal they used to get there - when the Alliance Expedition returned the favor and took the fight to Draenor, it was the Dark Portal that allowed them to do so. The Dark Portal nearly destroyed Azeroth when Ner'zhul ripped Draenor apart into Outland, and we used it to travel to that same Outland to do battle with Illidan Stormrage.
Yet for all we know about it, there's much we don't - was it Medivh or his dark corrupter Sargeras who actually instructed Gul'dan on how to build it? How does it function, what magics sustain it, and how does Warlords tease us with the possibility that there could be many methods to power such a doorway between worlds? Why did Ner'zhul's attempt to open more portals like it destroy his Draenor, and could the same fate befall the one we're traveling to - or worse, our own Azeroth? How dangerous is it to tamper with space and, now, time in this way?
Let us pursue the history of the Dark Portal for some answers. There will be spoilers for Warlords of Draenor in the post to follow.