As you walk down the tunnel you can see what the voice meant by the stench. You have crossed week old battlefields that smelled better than this place. You round a corner and come to a chamber when a very slight pillar of light illuminates the roots of a tree, as well as large rocks strewn about. On the roots of the tree grow mushrooms matching the description of what you are looking for. Make a spot check.
10 or better
SpoilerYou notice one of the rocks has a ring cemented onto it that is attached to a broken chain.
15 or better
SpoilerYou notice runes on the floor of the tunnel about five feet from the connection to the chamber.