A written work that hides its true contents behind a blood offering. The journal of Umin Ulthano, shelved in the Archives of Kulbak Prison, is one such book.

You see a rusty nail protruding from the binding and suspect the journal is a bloodbook, its true text only revealed when it drinks from a reader. - Backstory Codex - Bookworm (RPA-BC) 

In between the lines of near-lunatic text you recognize from the printed versions of the book, strange glyphs and scrawled lines of spidery handwriting appear. In the appearing blood ink, Ulthano writes of how he crafted his book at his forest retreat, and in doing so he opened up the path to all knowledge.

I looked to the stars and poured out my wisdom.
If I lose my mind, I lose my way.
I must call to those same stars.
They will come, and I will go to them.

As I completed the work the book itself vanished.
Now I too must go.
Too much of me is in those pages.
I looked to the stars… - Backstory Codex - Bookworm (RPA-BC)

Opening the bloodbook with Nuli at the Abandoned Watchtower reveals a cryptic message;

Together, holding your bleeding hands over the book, you cause the hidden writing to reappear, and match the glyphs Umin wrote in Kulbak to those etched beneath a window looking out above the forest. “Of course,” says the scholar. “It reads ‘Open eye, red sky.’ - Backstory Codex - Bookworm (RPA-BC)