Much-Maligned

Just finished reading BR:T. I might have missed this but what was Nagash's sacrifice of Mount Avalenor intended to achieve? And when did Celennar point this out? 'Here was the great sacrifice Nagash needed to power his theft: not a mortal soul but that of a living mountain. Its end would be enough; that much Hysh itself knew, and had spoken through the wisdom of Celennar.'