Forces of the Abyss
When the constellations are in alignment and the power of the Wicked Ones waxes strong, their infernal legions march forth. Cackling, spiteful devils march alongside muscled behemoths. They are the Forces of the Abyss and the world screams in their wake.
A great rent in the ground, the Abyss is many miles deep and countless leagues across. A pall of smoke hangs above the Abyss, so that it and all the lands about it are shrouded in unnatural, perpetual night. The empires of the Abyssal Dwarfs as well as countless orcs, goblins and other wretched creatures all exist there. Further down this hellish crevasse, the taint of the malevolent energies of the Wicked Ones becomes stronger, and things far worse than orcs dwell.
– lore snippet from 3rd Ed. Rulebook