From the website:
When greenskins gather for war, their Shamans build mighty idols of Gorkamorka. Some are formed from the rubble of the Orruks’ recent conquests, mortared with noxious Squiggoth dung. Others are built using strange stone quarried in the deep places of the realms, or even incorporate volatile shards of realmstone or shadeglass within their lumpen forms. Daubed with crude sigils, lit by roaring bonfires, the idols sit silent and menacing as the Shamans dance around them and the tribes bellow their war chants. The idols soak up ever more Waaagh! energy until at last the innate magicks of the greenskins react spontaneously with those of the realms themselves.