Grelvazub's diary is the book read by the children in The book of dark secrets.
One of the speculated causes for why Chris W. Smith was placed in a time loop was "read[ing] from the book of the dead." (Groundhog Daying deleted scenes). It is theorised that said-book refers to this diary.
Translations
While the diary is written in a mystical language, a boy was somehow able to read and translate the words into English.
Original language: Nilbursh qualarck mashtama. Chohorum frondurbel darshik. Balaigo olaazhi parnakam.
English translation:
This tuesday in possession class, I got called up to the board, but I felt something happening. You know. Down there. I think Tervina the water ghoul might have seen it.
I Grelvazub had it happen to me again. This time I was hovering in the tombs when I got a total stiffy. The minotaur lunch lady was right around the corner, so I-
I woke up this morning and I done it again: wet myself. But this time, I was at Skigdar the Neuromancer's house for a sleepover. I tried to hide my sleeping bag, but his mom totally found it. I know she knows what happened. Skigdar lives next to Tervina the water ghoul... ... and I know their parents are friends. I hope they don't tell Tervina. I think Tervina is just the sweetest... ... funniest... ... prettiest... ... scariest, most amazing being in all the worlds. Well, I could never tell her. But oh boy-