Invisible Sun: The Threshold - Rules of Play says:
Within the pages of The Threshold, you will find:
• New secrets, fortes, and creatures, plus new magical practices of all sorts.
• Secret degrees for all of the orders.
• An in-depth look at the life of a vislae, offering GMs and players a foundation and understanding for all characters.
• A detailed look at the mysterious Labyrinth, a final challenge faced by powerful vislae before they transcend to...something else.