According to an inscription of the Legion of the Dead, the Stone is balanced but not pure. That means besides its good attributes, it also contains the gangue, the waste and unstable rock which should be cut away. Subsequently, in order for the dwarves to prosper they need to face the gangue and carve it out of themselves.
Once the Warden collects all the pieces of the Legion of the Dead armor set as part of The Dead Caste quest, any party member who is wearing the full set can activate the Legion of the Dead Relic, an object located in the center of the mausoleum, the very room where the Helm of the Legion can be found.Activating the relic will summon the Gangue shade, a Boss-level ranked enemy. Once the shade is destroyed, its remains may contain a Chainmail. The encounter does not trigger a quest or journal entry, however, interacting with the Legion of the Dead Relic will unlock Codex entry: The Gangue Shade with or without the Legion of the Dead armor on.