Between Fulgrim, Ni and Arianna the group is able to locate the levers that allow the portcullis on the Tomb to be locked into place successfully. But this leaves the question of who it was that opened this tomb in the first place? It certainly wasn't the Undead as the Portcullis is a Cast Iron Model that has to weigh at least a Metric Ton when all is said and done, and from the appearances it doesn't appear as if it was designed to be opened from the inside of the tomb.