Line 3 - Even to the Promethean Serpents that revealed the truth of the Great Tree and to the fallen King of Terrors who waged war against its very essence.

Line 3 - "Even to the Promethean Serpents that revealed the truth of the Great Tree and to the fallen King of Terrors who waged war against its very essence."

To begin with, there is the Greek mythological character of Prometheus, who is often equated with the character of Lucifer.  Both Prometheus and Lucifer are presented in Mythology as figures who brought "light" to humanity and were somehow punished, or had fallen, because they had revealed secrets of the divine, or the gods, to human kind.  Prometheus had given fire to humans, among other things.  Lucifer is known as the "light bearer".  The question then, revolves around what this 'fire' or 'light' is about and how it corresponds with the ascension described in the first lines.  

In my studies, 'fire' or 'light' bears reference to the force of the divine that is closest to G*d.  On one hand there is 'fire' as symbolic as the 'highest' of the four elements, on the scale of ascension, and being somewhat analogous to spirit.  

On the other hand, 'fire' is also associated with being closest to G*d through the description of the highest form of angel, The Seraphim, or "fiery serpents".  [In Isaiah 6-6 the term is used to describe a type of celestial being or angel. The other five uses of the word refer to serpents].  The term "Seraphim" has often been translated as "the burning ones", so thus also connecting them to the property of fire.  Perhaps these then are the Promethean Serpents being described.

The Great Tree:

The tree is a common theme in many world mythologies, including the story of Genesis and the tree in the garden of Eden, the Norse Tree of Yggdrasil, and the tree of Huluppu in the story of Gilgamesh.

With regard to the introductory line making reference to Jacob's ladder though, I'm inclined to consider this tree as the K. Tree of Life.  Here is an image of the serpents corresponding to the various points of the tree (although in standard tradition the serpent on the tree would dictate the path of ascension, and the path of descending would be the lightning flash from above).  

"The Fallen King of Terrors".  Here it gets a little tricky.   

I've read before that some considered this as another name for Rex Mundi, "King of the World", or King of the Earth.  There is text that appears later in book which might appear to support this, when it says "For with the formidable jewel that fell from the crown of Rex Mundi is truth known. Look to the armoured King of Terrors...".  

There is, however, no other source outside of the book that I've discovered to support this directly.  

The other problem is that the text appears to allude to the possibility that, like Lucifer, the King of Terrors is a fallen angel.  However,  there is a paradox with this association (as jlockest pointed out here: http://tweleve.org/maranatha/25594-once-more-unto-breach-dear-friends-once-more.html). Lucifer and Prometheus are known to bring fire or light to humanity, and thus revealing the truth of the Great Tree, however the King of Terrors actually wages war against it.  

Outside of this, there is another reference to "The King of Terrors" that I've found.  One that involves the figure of Death as a skeleton and wielding a scythe.  This image happens to come from Rosslyn Chapel.  


The exact words of "the fallen King Of Terrors" can be found in a book called "The Bow in the Cloud"https://books.google.com/books?id=hnQ9AAAAYAAJ&pg=PA209&lpg=PA209&dq=%22fallen+king+of+terrors%22&source=bl&ots=VWstk3cFnH&sig=760eTtMXdEdmkTZ6WJsC7Q0AsZs&hl=en&sa=X&ved=0CB8Q6AEwAWoVChMIsLm0kZz6xwIVQSUeCh2ykQkI#v=onepage&q=%22fallen%20king%20of%20terrors%22&f=false.  

In this passage it refers to the King of Terrors being re-instated by those who morn the dead and cause disloyalty to their "incarnate, dying and risen Lord".  The text is of course a reference to Christ, but I'm reading more into it.  The subtext to me is speaking that the King of Terrors is the one who tries to prevent those trying to ascend the Tree. Thus has the King of Terrors waged war against it.  If to ascend is to understand the "Immortal Knowledge" that the Guardians of heaven knew, then the King of Terrors (Death) threatens to prevent this from happening.

"If to ascend is to understand the "Immortal Knowledge" that the Guardians of heaven knew, then the King of Terrors (Death) threatens to prevent this from happening."

I believe this is also described later in the text: "Look to the armoured King of Terrors see him cast away a pilgrims staff and take the balanced shape of a staff of death a sword with this power in the mortal realm he seeks to slay the innocent before they receive the gift of freewill."

The innocent are those who have not yet received the "Immortal Knowledge". Thus perhaps this knowledge, that is being talked about, is the knowledge of Freewill.