The Nanuak is the world soul, something that the Raven Clan Mage, Saeunn, once described as a 'river with no end'. Each living thing has a part of the Nanuak in him and one of his three souls; the only one that can never be detached from him, even in the event of death; is the Nanuak. It is presumably what allows the various sentients of the Forest to communicate with one another and even discover the ways of others.
At some point, the Nanuak can form into something very powerful which becomes visible and tangible. Froth in rivers and waterfalls are such exemples of the Nanuak's manifestation. But the Nanuak can also form point of far greater power than a mere waterfall. During his quest to destroy the demon bear, Torak was told by Renn that he needed to find the strongest parts of the Nanuak to appease the enraged World Spirit, the only one who could destroy the powerful demon. Torak found the three parts of the Nanuak: river eyes, a stone tooth and a lamp in the hands of a dead man. Presenting these strong pieces of the Nanuak to the World Spirit atop its Mountain allowed Torak to destroy the bear.
These parts of the Nanuak were so strong that they could absorb the part of the world soul in other living beings if they touched them with their bare skin.