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
- 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.