Back-story

The text was written as an instruction/recipe book for bringing about the invasion of this world by creatures from the next. It was designed to be read aloud in a church-like setting, taking the gathered devotees through a series of synchronized transformations throughout which the invading creatures play-test their human hosts, come to understand the physical realities - the physics - of our world, before fully transforming their human shells into an appropriate simulacrum of themselves. 

 The last stage involves the initiation of a full scale invasion. The book is ancient and it's purpose has been semi-successfully fulfilled twice previously. The first involved a fairly sizable gathering of people. The transformations were undertaken but were interrupted by authorities. Many lives were lost in the ensuing bloody battle but the invasion was halted fairly on in the ritual. The second involved a lone individual who brought the entire process to fruition but was destroyed – along with the few creatures who'd utilized him as a gate

A failed exorcism was in reality an interrupted transformation, first stage. The “possessed” individual was institutionalized after she killed the priest who'd attempted the exorcism. Her utterances and mild physical oddities – the first stage is mostly a mental transformation – made their way, through psychological journals and communities, down into occult circles and the attention of someone familiar  with the book and the previous attempts at its enactments, and thwarting. 

With the first-stage victim as evidence of sorts, this person garnered the ear of some government support and put together a plan for an organized response, albeit one known of by only a handful of people and considered as anything other than nonsense by less. The occultist and his small team, and the possessed woman are all characters late in the game. The occultist and his contingency plan are the government's next-to-last response (the final response being the total annihilation of the building and everyone inside it). His team and ideas are not thought of until word of the insanity in the quarantined building reaches the right ears, and those in charge have been convinced that insane conditions warrant an insane response.

The possessed woman has been interrogated mercilessly by the occultist and his team and is kept as their prisoner/insider source of information. Over the years she has formed a peculiar relationship with her captors. They are old enemies and completely different forms of life, despite her outward appearance, but they can communicate on a rudimentary level, having spent enough time attempting it. The creature that inhabits the woman's body has learned how to aggravate, poke and prod her captors the way a parrot might learn that certain vocalizations elicit certain responses from it's owner. It has tried unsuccessfully to coerce and force a weak willed character to read the book to it and further it's transformation.

The savant was brought into the building to make a high quality audio reproduction of his recitation. The lengthy text of the book he's memorized, the equivalent of the necronomicon, is believed to have been destroyed after his peculiarly extensive memory had absorbed it.