Deep beneath Oa he finds the Black Lantern logo, and beyond that, the Book of the Black that the evil Guardian, Scar, had been using. The first page we see is of Guy and Kyle Rayner fighting, and then when Guy shows up, we see Lyssa Drax still stuck in the pages.
Ganthet wants to analyze the contents, but Guy is content to just bury it under ground in Oa and not go through it all again.