The Song of the Guardian, or the Punishment Cell Monstrum

Sometimes Daren tells visitors to the Hellsong Tavern about how Malik became the keeper of the library. After the demons found the keeper's helmet, they wandered through the halls of the old vault for a long time until they found a folio with a keyhole instead of a spine. Malik decided that the book could hide important secrets, since it was locked, so he opened it with a skeleton key.
 
The folio opened with a slurping sound. Something crawled out of it — a clot of torn chapters, living ink and embodied chaos. The monster rewrote reality on the move: the floor became text, the cabinets became sheets of paper. The demons ran, stumbling over the scrolls, but the monster still caught up with Malik.
 
In the hands of Daren's friend was a guardian helmet. He put it on his head so that it would stop interfering, and was already reaching for his sword. But then his eyes flashed with yellow light, and his body convulsed. The demon froze for a moment, and then grabbed the creature with his bare hands and turned it into pages of books. He put the parchment sheets, which had previously been a monster, back into the folio and locked it.
 
Daren was still alive. And Malik slammed the book shut, sat on it and said, "Now I'm the new keeper."