Vice Hierarch Zenya:
In the Imperial Tomb, there are four Sepulchers. I think you need an Entrance Pass to get inside the sepulchers. The monsters collected these Entrance Passes while collecting the relics. If you need a Entrance Pass tothe Sepulchers, get a mission to collect Relics of the Empire. As you kill monsters for their Relics, you will run across Entrance Passes.
If you want to know more about the four Sepulchers in detail, find the Nameless Spirit deep in the tomb.