From the announcement:
Arkham is under threat once more, but this time, evil is not emerging from beyond the veil that divides worlds. It comes from within the city itself. The Order of the Silver Twilight is a powerful, esteemed society within Arkham, but their secretive nature naturally incites your curiosity. What are they really up to and what are their motives? Fantasy Flight Games is excited to announce the upcoming release of Sanctum of Twilight, a new expansion for Mansions of Madness Second Edition, now available for pre-order at your local retailer and online through our website here!