The Mass Grave is a small dungeon in the Old Tristram Road in Act I of Diablo III. It can be accessed by entering the hole in the ground on the way to the Tristram Cathedral. It should be taken into consideration, however, that it is not there in every game.
Upon entering, the player will be ambushed by six Risen. There is nothing special about this area but a player can clear it in a few seconds to get some gold, and it needs to be explored in order to get the achievement No Stone Unturned.