A basic outline of the lore has already been created, and you can help add to it if you wish: 26 years ago, a Dark Portal was opened in the Blasted Lands. Through this portal, the Orcish Horde came and marched towards the unaware humans.
The nearby town of Sunnyglade, located near the Last Guardian's tower of Karazhan, heard the news about the Portal and a possible invasion. A force of sages, magi, trackers, beastmasters, priests, mercenaries and knights was created to travel to the Dark Portal and see where the Horde had come from.
Entitled 'Expedition of Grand Hamlet', it was originally led by Archmage Mortagar of the Kirin Tor. Soon after entering the Dark Portal, their camp in Draenor was ambushed by Orcs, and the majority of the expedition were slain. The mercenary captain Anarion Bloodrage was one of the survivors capable of leading and took the remainder of the Expedition away from the ambush, ending up in the beautiful lands of Nagrand. There, they decided to set up a camp that would shelter them from the Orcs who pursued them. A magical shield was set up to conceal the location of the hidden camp. Weeks passed, however, and many of the people were too injured to continue moving. The shield was sustained, and those able gathered resources from the nearby pastures. Over time, the people got used to the lifestyle and the small camp became a substantial city, full of traders and merchants. The members of the expedition eventually had children inside the hidden city and a new generation grew. Anarion rose to power until he became the Emperor of the Gryphonheart Empire, named in honor of the Alliance.
26 years had passed, and the once-young and proud warrior became an old man. His pride was now his son Decimus, a warrior of only 22 years, but already of 8 years battle-experience. Decimus enjoyed a comfortable lifestyle, good food and many women, but was always the first rider to charge in battle against the Gryphonheart's enemies.
Lord Illidan's forces traveled the Outland, conquering place after place until they by accident stumbled upon the magic shield protecting the city. Without question, the city was attacked, and soon overwhelmed, for it never needed physical walls when it had magical ones. But now all the magic was drained, the city was left defenseless and fell within minutes.
The survivors escaped, but Anarion stayed behind, guarding his people's retreat. He was never again seen, and presumed dead.
Decimus took control of the remainder of the people, taking all those who survived away from the city's ruins. It was by blind luck that they found the Dark Portal. They had no choice but to go through to Azeroth, seeking revenge and justice for their fallen comrades.
The Gryphonhearts set off to the only place they knew for sure to be safe - the great Nation of Lordaeron. On their way they were surprised to find many camps of the green-skinned Orcs, and almost feared the worst. However, they were even more shocked to find that many of the great cities in Lordaeron had been destroyed and were now full of the walking dead. They were accepted, strangely, by the shadowy, quiet humans in Pyrewood Village, although after a night there, they figured that they'd best spend their nights in Ambermill. Every evening, at around quarter to nine, a mass evacuation starts, until Pyrewood Village is safe again in the morning.