If you borrow the Tome of Conquest at the Librarian O.D. and head to a room located at Glacial Tomb (more specifically two rooms to the left of the area's warp portal), you will be met by the Librarian himself, ready to enact punishment for not returning his valuable book! he is fairly strong, but beating rewards the player with a shard that allows for stopping time. Also, if you return to his library after beating him, he will be wearing a costume mask and will let the player "pillage" his books.