Oh, shut up and help me remodel the Magical (element) page!
Fog prevents the use of magic, and Shell increases resistance to magic, while Deshell makes targets more vulnerable to magical damage. Faith increases the power of magical attacks used by the target, and decreases with Defaith. Magical attacks tend to add higher percentages to the chain gauge than physical attacks, but also cause the gauge to decrease more quickly than their counterpart.
This makes magic rather impractical for initial chain building. However, magic is useful for staggering foes after their chain gauges have been stabilized through repeated attacks, and even more useful for post-stagger chain building.
Magical returns as an element, and the same mechanics in Final Fantasy XIII apply here as well.
Magic or sorcery is an attempt to understand, experience and influence the world using rituals, symbols, actions, gestures and language that are believed to exploit supernatural forces.