Iggwilv wrote the Demonomicon as a reference work on the Lower Planes and included a great deal of her own research. Over the years it has been stolen and copied several times. One of the more useful copies found itself in Aerth. It was not copied directly from the original, but has a very powerful enchantment on it that instantly copies anything written into any book called the Demonomicon. Everything from highlighting to footnotes to actual sections written by Iggwilv or anyone else is instantly copied into the book. Because of the nature of the book, it is incredibly long and more or less incomprehensible. A quarter of the book is not the actual writing copied, but another book called the Praxus that explains which of the sections and scribblings are gibberish and which are good.
This particular Demonomicon found its way into a dwarven hammer ship where it was divided into four sections and those pieces were scattered across Aerth so that no one could wield its profane power.
One of the pieces was owned by an evil gnome wizard named Skullshank, and came into the possession of the party along with his observatory. A second piece still resided in the dwarven hammer ship and was being held by a group of fiendish formians until the party liberated it. The third was in a far away crystal sphere called Trevyrn on an abandoned colony, and the party traveled to Trevyrn and braved the wild magic their to get it.
The last piece is currently being held by the evil cult of He Who is Divine and Righteous Might. Once the party gains that final piece, the ritual can be done and Aerth can be saved from a horrible fate.