Originally they were natives of a large island, south of Terra and east of Highwind. They had a tribal culture based on hunting, fighting and asserting dominance over one another. After the failed invasion of Heartland Luther the Wicked had the idea of capturing and using the orcs as shock troops and fodder.
He had his people try to train, magically enslave or otherwise control the orcs but no one was able to tame them. No form of mental control worked, and even orcs born in captivity were too wild to control. Eventually Luther's people came to him in failure.
One man managed to get himself spared; an animal trainer who gave Luther an idea. He said that they could capture more orcs and ship them off into the southwest of Heartland. Toss them spare weapons and equipment and sail away and let the orcs weaken their enemies.
Luther saw it as a genius idea; great potential damage to his foes and tying up Heartland forces while not needing to invest his own forces. He ordered the orcs collected with two military camps made on their island. They shipped the monsters out , using fog as cover as often as possible. They dropped off five hundred thirty orcs just six years after the failed invasion. More would be delivered over the years.
They had captured a number of orcs from separate clans and so infighting was common. Eventually the orcs formed clans and started to find their own territories inside the forests. Those that couldn't maintain a hold on territory were forced to seek new forests to live in. Some moved to the forests in the east, others to the north.
It was two years later that the orcs were first noted by the people. At first there was no contact; the orcs kept distance; more focused on establishing camps and settlements in the forest. But soon contact came; lone travelers, the odd raid, but nothing as significant as what they had suffered in the past.
Still recovering from the invasion, the army didn't have the strength to fully stamp the orcs out. While not as smart as humans the orcs were cunning and strong and were born survivors. They managed to either fight back or retreat to new locations.
Even after the Heartland army recovered they have never been able to fully stamp out the orcs, keeping them mostly contained in the southwestern forests. The orc clans are unorganized, fractured, and are as liable to fight each other as anything else, preventing them from being a bigger threat than they already are, but they are still a danger to travelers.
They have not made any attempt at negotiation, let alone seem receptive to the concept.