Rift of Aln
The Rift of Aln is a vast, deep fissure in a small corner located deep within the Emerald Dream, from which it is thought by the druids that the Dream itself originated. The druids also believe that it bleeds into the Twisting Nether and the Great Dark Beyond. Players enter the Rift of Aln through a Maelstrom-like portal within Lake Elune'ara, in the Emerald Dream's counterpart of Moonglade.
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.
Warlords of Draenor
This section concerns content exclusive to Warlords of Draenor.
During the War against the Iron Horde, Archimonde held a fragment of power from the Rift of Aln, and the revelation disturbed Malfurion greatly. Believing it to be a terrible portent, he set off for Val'sharah, where he was first trained as a druid.
This section concerns content exclusive to Legion.
Resolved to rescue Malorne from the Nightmare, Remulos, the Archdruid and the druids of the Dreamgrove devise a way to reactivate the portal to the Rift of Aln that lies abandoned in a dark corner of the Emerald Dreamway. The druid hero and their allies reach the Nightmare and find it warped into a recreation of Malorne's memory of the War of the Ancients. The druids must work together to destroy the nightmare demons and help Malorne relive his fatal encounter with Archimonde, but ensuring that Malorne survives this time.
When dispersed, the Rift of Aln seems to have been located on top of a titan-made dais that Cenarius stands on.
Portal to the Rift from Un'goro Crater's Nightmare.