You need find 6 pages with the history of the Hidden. We will tell you where to find them.
The screenshots below show the locations of all the Invisible Offices in different cities / regions in New England.
Shoot the hanging bag of stones.
Go southwest of the Ealdorman House in Colchestre. Climb wood and stone and look for a destructible floor. But you will need a red oil jug. Look for it in the nearby market.
Northeast of the Lunden amphitheater. There will be a mossy wood ring next to the columns. Climb up and dive into the hole. Get out of the water and climb up.