In the worlds of Cocoon and Pulse in Final Fantasy XIII, the powerful fal'Cie have the ability to brand humans as l'Cie. When branded, the l'Cie are bound to a specific task that the fal'Cie assigns them called a Focus. Failure to accomplish the given Focus results in the l'Cie becoming a Cie'th. However, l'Cie that successfully complete their Focus transform into crystal and enter a state of stasis.
The amount of time that l'Cie have to complete their Focus is determined by the state of their l'Cie brand. As time passes, the appearance of the brand changes, first growing arrow-like points, and then an eye. When the eye is fully open, the l'Cie has run out of time. The rate at which the l'Cie brand changes appearance varies between individuals.
The actual Focus that the l'Cie is charged with is given to them in the form of a dream or vision. It is the responsibility of the l'Cie to accurately determine the task at hand; a l'Cie may pursue what is believed to be the proper Focus to the end only to realize too late that the fal'Cie had other ideas in mind. It is also possible for multiple l'Cie to be given the same Focus, as is the case with the main characters of Final Fantasy XIII. Additionally, fal'Cie have the ability to turn humans into l'Cie without granting them a Focus. Doing so immediately turns the unfortunate l'Cie into a Cie'th, as they cannot complete a Focus they are never given.
If a l'Cie manages to complete his or her assigned Focus successfully and enters crystal stasis, they may remain in that form indefinitely. Fang and Vanille, having previously completed a Focus hundreds of years prior to the beginning of Final Fantasy XIII, remained as crystal for the duration until the fal'Cie awoke them and once again assigned them a Focus.
Normally, humans must rely on technological devices in order to use magical abilities. Those branded as l'Cie are given the innate ability to use magic of curative, offensive, and defensive capabilities. In Final Fantasy XIII, the player's party learns new magic abilities as they spend Crystarium Points to advance along the Crystarium.
In the event that the l'Cie becomes lost, aimless, or filled with despair, an Eidolon will come forth from within them and attempt to finish them off. If the l'Cie is able to gain mastery over the Eidolon, it becomes a powerful ally, and the victory may give the l'Cie the will to continue on in pursuit of completing the Focus.
l'Cie in Cocoon Society
In Cocoon, Pulse l'Cie, or those branded as l'Cie by a Pulse fal'Cie, are reviled as outcasts that threaten the stability of Cocoon and are hunted by the Sanctum, Cocoon's governing body. By contrast, Cocoon l'Cie, or those branded by a Cocoon fal'Cie are considered heroes if they manage to complete their Focus.