The Rift of Aln is a vast, deep fissure in a small corner located deep within the Emerald Dream, from which is thought by the druids that the Dream itself originated from. The druids also believe that it bleeds into the Twisting Nether and the Great Dark Beyond.
Gazing into it, Malfurion Stormrage saw it as a bottomless chasm which radiated with primeval energies that even he dared not investigate. Indeed, the very rift itself seemed half-dreamed, for it had a surreal quality to its expanse and to the archdruid now and then seemed to ripple as if ready to fade or change.
Malfurion and Tyrande Whisperwind cleansed the Emerald Dream everywhere, except in the rift where the Nightmare stubbornly held firm. There was no point to keep pushing, as the people of Azeroth still needed the couple's help. Thus, Malfurion sealed off the vicinity around the rift, he and Tyrande silently agreeing that the day had been saved, and this was another war that would have to wait for a better time.