25,000 years ago, the Bugbears lived on a lush forest planet they called Far'esh. They lived there in peace and had many magical abilities: one of them was the ability to commune with the spirits of their long departed ancestors. These were not mere visions; this was actual communication with the dead. Such power gained the attention of the Nathrezim who came and began to enslave and experiment on the Bugbear race.
From the Bugbears, the Nathrezim acquired their deadliest ability: their power to raise the dead. From that point on, they began experimenting on worlds they conquered with the Burning Legion. 10,000 years ago, during the War of the Ancients, such experiments were observed by the Orc Broxigar. He stumbled into a Night elf village where the Nathrezim were raising Night elf corpses. These undead were not as powerful as the Scourge we know today; they were easily dispatched, but Broxigar knew that this was the first signs of the Future Scourge. He quickly dispatched them and the two Nathrezim raising them.
While the Bugbears remained as slaves by the Nathrezim, they slowly devised a plan to escape. Of course, they had no remained entirely pure during their long years of enslavement. The energies from the Twisting Nether changed them over time in ways the Nathrezim could never have expected; they were able to teleport short distances, but not without cost. 500 years of practice and planning, warped their forms and started to corrupt their bodies. They learned forms of combat in which they secretly practiced and honed. The Nathrezim grew suspicious of their activities. The Bugbears decided that it was now or never.
They launched their attacks on the Nathrezim. Once they teleported to the Nathrezim planet of Xoroth, the Bugbears fight back, but at the same time, send out psychic distress calls to any who would offer their aid. Shortly after, they felt the light surround them. A voice cried out, "We have heard your pleas, we have come to aid you". It was the Naaru. The Naaru have come to aid the Bugbears and with the forces of light behind them, the Alliance and Horde included, "We must prepare for the Light's retribution on Argus and Xoroth" (Patch 6.1).
The Bugbears were capture a Nathrezim portal which leads to Argus and contact the Horde. Entering the planet, the Bugbears prepared to face the enemies from the Burning Legion. Instead, they faced potential allies, known as the Horde. The Horde assisted the Bugbears in finding a land on Argus as their primary staging ground / home. At last, they found a land and started to build their capital city and major civilized resources. Hoping to reclaim and restore Argus from its corruption, the Bugbears were welcomed to the Horde.