The Cabot journal was a journal kept by Sebastian Cabot, an explorer who spent time in Haven around 1496.
In the See No Evil episode Vince Teagues explains to Duke Crocker and Nathan Wournos that Cabot's Journal is written in a very old ancient Mi'kmaq language that is going to be very hard to decipher to help them with defeating Mara and it is known to have a map or the location of The Thinny in Haven.
Journal page on horseshoe crab harbinger
One page had a drawing of a Horseshoe crab with human eyes and included the warning:
The Horseshoe Crab
It also had fragments of Mi'kmaq hieroglyphs saying:
Journal page with Salvation/Doom warning
Another page included the warning:
What was once your Salvation
is now your Doom.
- Limulus polyphemus is simply binomial nomenclature for the Atlantic horseshoe crab.