The codex is the parchment icon on your Journal screen and is the repository of important knowledge uncovered in the game. It falls into 10 categories: creatures, items, magic and religion, culture and history, characters, books and songs, notes, spell combinations, control, and quest-related. As you unlock a codex entry, a scroll appears in the appropriate category and you can read volumes on the various topics. Check it regularly for information, especially if you need a clue to a puzzling mystery on your current quest.