Spoilers below, ye be warned. In bold I am dropping some serious story spoilers and speculations and I'll back it up as best as I can.
In Legion it is revealed that Alleria and Turalyon, upon stepping through a portal to the Twisting Nether at the end of WC2 have been bouncing around time and space, that they have been battling the Burning Legion for a Thousand Years.
Vindicator Boros:
The vessel...it senses young Arator's presence!
High Exarch Turalyon:
If you can hear this, your mother and I have an important message for you.
Alleria:
For a thousand years, we have been hunting the Burning Legion across the stars.
High Exarch Turalyon:
We do not expect you to forgive us for leaving, but we did it out of love for you. To ensure your safety and the survival of our world, we chose to make this sacrifice.
Alleria:
We failed in the final battle, and it is unlikely we will survive. My son, we have one final message for you and the heroes of our world.
High Exarch Turalyon: The Burning Legion will come in full force to Azeroth. These armies are led by Balnazzar, Gul'dan and Kil'jaeden. These three must fall before you can hope to defeat Sargeras.
Alleria:
Azeroth is the last light of hope, Arator. Fight, or all is lost.
High Exarch Turalyon:
Farewell, son. May the Light guide you.
Arator:
My parents risked much to send us this warning. We must act.
Alleria and Turalyon, presumably along with a thousand years worth of gathered forces, have been hunting the Burning Legion throughout the Twisting Nether. By travelling through the Twisting Nether, they have not been aged (as the Nether is timeless).
Alleria was captured, interrogated, and lost her bow, but Vereesa concludes that she must have escaped.
This happens during the Marksmanship Hunter Artifact Quest-line.
Alleria and Turalyon may have visited Alt. Draenor some time in the past 1000 years.
In Warlords of Draenor, we encounter the
Cowled Ranger, who sends us on a quest to confirm her suspicions that Alleria is somehow on the Alternate Draenor.
Her suspicions are
confirmed true, the existence of the
Silver-Lined Arrow is physical evidence of Alleria's presence on Alternate Draenor.
But it doesn't confirm when.
Speculation: Alleria and Turalyon arrived on Alt. Draenor hundreds of years prior to WoD, and the Apexis left with them.
So I believe that Alleria and Turalyon encountered the Apexis on this ancient Draenor, and were able to convince them to join their cause.
The Apexis disappeared mysteriously at the height of their power, leaving behind technology that rivals that of the Draenei. The only clue to where they may have gone is with the
Apexis Scroll.
"
This battered scroll case has corroded shut, and the scroll within is barely readable with very careful examination. These appear to be plans and measurements for a glorious "Temple in the Sky," written in an ancient hand. It's unclear whether such a splendid structure was ever built.
A Temple in the Sky? Sounds like a Dimensional-Ship to me.
So Alleria and Turalyon's forces, aboard an Apexis Dimensional Ship, hunted the Legion for thousand years.
But let's not stop there:
Who could they have picked up over their 1000 year crusade?
For starters, Alleria and Turalyon were said to have disappeared along with a small force of Men and Elves. A thousand years is a long time for Half-Elves to emerge as a people of their own, with culture that is influenced by both races.
Legion introduces a race called the
Aldrachi, who were wiped out by the Legion, but there is a possibility that some were rescued by Alleria and Turalyon, adding them to their forces.
Finally, it would be difficult to traverse the Nether for 1000 years without encountering some Ethereal who would be interested in joining their cause.
Wrathion probably knew that AU Draenor was where Turalyon and Alleria ended up at some point in history.
Not sure how, but I wouldn't put it past him.