The Horadric Staff and The Arcane Sanctuary are both needed in order to reach Tal Rasha's Tomb where Duriel is.They can be seen as prereqs for the last quests of each act In these cases, these quest must be finished in order to reach the area the act boss is in. Some acts have other quests that are necessary to complete. Sisters to the Slaughter: Kill Andariel.Each act has at least one quest that must be completed in order to progress, and that is the boss kill quest that requires the player to kill a Great Evil.