The Ossuary (Act 10) is an area in Act 10. It is connected to The Ravaged Square. Vaal side area has a chance to spawn. There is a Trial of Ascendancy in this area.
A small sub-area called the Forbidden Archive which contains an interactable object called "Heretical Tomes". Clicking it will drop the quest item The Mortinomicon Exitio Immortalis.