The Infernal Gate is a magical portal that was created by the Prime Evils during the events of Diablo II. It is located in the Durance of Hate, Mephisto's prison under Travincal. The Archangel Tyrael knew of its existence, as he hinted it to Marius in Tal Rasha's Tomb. During its creation, Mephisto spoke the words of the Prophecy:
"And the Evil that was once vanquished shall rise anew. Wrapped in the guise of man shall he walk amongst the innocent and Terror shall consume they that dwell upon the Earth. The skies shall rain fire, and the seas will become as blood. The righteous shall fall before the wicked, and all of Creation shall tremble before the burning standards of Hell!"
After this, the Dark Wanderer completed his transformation into Diablo and stepped through the Gate.
Current Whereabouts[]
At this point, it is unknown whether the Gate is going to make an appearance in Diablo III. However, since there was no word about it after the events in Diablo II, it is possible that it is still present in the Durance of Hate, active or inactive.