The Waaagh is the great, boiling phenomenon at the heart of the greenskin race — at once a war, a migration, a religion, and a tide of crude psychic energy that carries the Orcs and Goblins to conquest across the world. It is the closest thing the greenskins have to a god and a purpose, and the terror of every civilised realm that borders their lands.
A Waaagh begins when a warboss of sufficient strength and cunning cows the feuding tribes into following him. As orcs gather and fight, they generate a psychic field — a shared, ferocious will to battle that grows stronger the larger the horde becomes, feeding the greenskins' innate magic, making their shamans crackle with green energy, and driving the whole mass into a self-sustaining frenzy. The bigger the Waaagh, the mightier and more reckless its warriors, until the horde rolls forward like a green avalanche, smashing everything in its path.
Such hordes are born most often in the Badlands and the mountain wastes, then pour into the lands of men and dwarfs, and only exhaustion or the death of the warboss can halt them — for a Waaagh that loses its leader collapses as swiftly as it rose, the tribes falling to squabbling among themselves. To the greenskins, none of this is calamity; it is simply the best of all possible lives — a good fight, a big mob, and the roar of the Waaagh carrying them on to the next.