From Destinypedia, the Destiny wiki
Arcadian Valley, Nessus
Vex Fallen Cabal
Chamber of Water The Hollows The Tangle
Watcher's Grave is a location in the Arcadian Valley on Nessus. It is home to the Loyalist Barge that houses Werner 99-40, where you can collect Imperials, which can be used to purchase upgrades for the Chalice of Opulence.