20 years since what?
20 years since Turalyon](http://wowwiki.wikia.com/wiki/Turalyon) and Allerya went missing. This happened some time between vanilla and BC, when the Dark Portal opened leading to Draenor (pre-Outland) and Turalyon, Allerya, Khadgar, etc went in.
After they destroyed the Dark Portal (from Draenor side) they were still there when the planet was collapsing. Khadgar created a portal in desperation and they found themselves on Argus. Khadgar (in Shattrath) explains it quite nicely:
I've heard your name spoken only in whispers, mage. Who are you?
I am Khadgar, former apprentice to the fallen Guardian, Medivh. I'm sure you've heard his name before.
It was I who discovered that Medivh had been possessed by the Evil One, Sargeras - and that he had opened the Dark Portal between this world and Azeroth. Though I honored my Master, I was forced to slay him and put his tortured soul to rest.
I fought alongside Lord Anduin Lothar as we drove the Horde from our lands in Azeroth. As the heir of Medivh's legacy, I could do no less.
When the call finally came to invade this grim world - and put an end to the orcs' evil forever - I stepped forward along with my friends Turalyon, Alleria, Danath and Kurdran.
Gossipgossipicon I see.
My comrades and I called ourselves the Sons of Lothar - in honor of the greatest, most selfless man we ever knew. We gave our all to defeat Ner'zhul and end his madness.
As this doomed world fell apart around us, we shattered the Dark Portal to prevent any further harm from befalling our beloved Azeroth.
Gossipgossipicon What did you do then?
Trapped in this broken wasteland, we did our best to survive. Over time we sensed that a new evil was closing in around us from the Twisting Nether. More foul than the dark orcs - it was the Burning Legion itself that sought to take hold of this... Outland.
Gossipgossipicon What happened next?
While my comrades held the line against the encroaching darkness, I used my magic to reach out into the Great Dark Beyond. There, I sensed beings of immense Light - of unfathomable purity.
It was A'dal and his naaru I had found. It was nothing short of... a miracle.
Gossipgossipicon I see.
<Khadgar smiles to himself>
I convinced A'dal and his brethren to return here with me. I believe that only their power can drive the Legion from this universe. But as you will no doubt find - their power must first live in our hearts.
To combat the darkness, we must become beacons of their eternal Light.