From the introduction:
It is a truth understood by all who delve the ruins of the world that no Brother of the Deep Guild is ever left behind. Any Brother who does not return from a commission is searched for; to be either found alive, or to have their remains returned to the light of day. All Brothers in death find their rightful place within the Catacombs of the Fallen beneath Das Vallendor, and all of the Brethren understand that a search begun will not end until the Lost have been found. Such a circumstance befell Dannam Tull, a Scholar of the Guild and descendant of Dellig Tull, greatest of the early explorers of Arborell. In the story of his recovery can be found a true legend of the Deep Guild and the uncovering of the secret of the Tor'dreyel.