The Book of Sight is a magical artifact of great power, most notably the power to create a new Dark Lord. There have been many Dark Lords over the history of Foon, and whenever one is destroyed it is considered inevitable that another individual will find the Book of Sight and transform.
The details are understood Foon-wide:
"The Rhyme of the Dark Lord" (as given by Spintax the Green)1
He who holds the Book of Sight
When the moon is drained of all its light
Shall then be ruler of the night
Master of the dark!
According to Baron Ragoon, the book was created by the Goddesses of Foon as a place to hide their secrets and mistakes from the creation Foon2.
The first being to be transformed into a Dark Lord was a man named Ted in the village of Gunder. However he was quickly defeated by his neighbor with a hammer to the head. This incident is commemorated by the Winter Solstice holiday of Gunderstein.3 Other previous Dark Lords include a carfoon released by a student of both painting and magic,4 a warrior prince who lived 3000 years ago,5 and the first king of Grimfallon.6
The current Dark Lord is a corrupted wizard.
Tearing a page from the Book of Sight can free an individual from any magical bond or pact they have with the Dark Lord, so pages can sometimes be found with former minions who have rebelled.
Season 3 spoilers: