Mysterious chronicle (part 19) is a book found only in Daemonheim. It may appear in front of the dungeon-ending ladder after killing the boss monster.
It was written in the year 1730 of the Fourth Age. Bilrach has reached a new area weak to his portal magic. He likely is referencing the Occult floors as he was previously on the Furnished area. He plans to create a portal network which can be used to dump refuse in another realm and search for powerful creatures to guard his dungeon.
Year 1730 of the 4th Age:
A new depth has been reached and it is weak to my magic. Using portal magic should be safe enough here. If the portal network is successful, I can not only dump our waste and effluence into another realm, but I can search for powerful creatures to enslave and defend my great work here.Zamorak be praised!