Schemes of the Lich King
Based upon the scant knowledge that has been found from the agents of the Diabolist, it seems that something she has been looking for is an obscure arcane artifact known as The Dark Tapestry. Few people seem to know or understand what this is other than the Diabolist, herself. Even her agents, which seize every chance they can to find out more about it, seem to have no real knowledge of what it is or even why she wants it. However, signs point to whatever it is being associated in some way with Darkveil.
It is unclear whether or not this Dark Tapestry is even a physical item or whether it is something like a metaphor. Only time will tell.