"Some 1,300 years ago, a group fal'Cie constructed a paradise for humanity: the shell-like city of Cocoon, which floats high above the surface of Pulse under the power of the Cocoon fal'Cie Orphan. Then, the Cocoon fal'Cie created life forms and machines for Cocoon's inhabitants to use, and humanity flourished. A war was fought between the fal'Cie of Pulse and the Cocoon fal'Cie, and Cocoon prevailed in what was eventually known as the War of Transgressions. However, fear lingered in the hearts of the people of Cocoon, for the day another invasion might come from the world beneath again.
Although most Cocoon citizens have never seen Pulse with their own eyes, they have been told that it is a dangerous place that has strange effects on those who venture down to its surface. Consequently, anyone who is discovered to have been in contact with anything from Pulse is immediately subjected to quarantine and exile by the theocratic government of Cocoon, known as the Sanctum. The Sanctum enforces this policy with its strongest military branch, PSICOM.
As Snow leads the resistance group, NORA, in an attempt to stop the purging of civilians, the mysterious Lightning fights her way past PSICOM soldiers with the aid of Sazh to find a Pulse fal'Cie, Anima, who turned her sister, Serah (who is also Snow's fiancée), into a l'Cie. Through a chain of events, these three, along with two exiles, Vanille and Hope, are forced by the fal'Cie of Pulse to become l'Cie, and with that became enemies of humanity with the Focus of bringing about the downfall of the Orphan, who provides the fal'cie Eden with the energy it needs to run Cocoon. Slaying Orphan would supposedly result in Cocoon's demise."

"The concept for Final Fantasy XIII's battle system is to maintain the strategic nature of command-based battles. The system stemmed from a desire to create battles similar to those found in Final Fantasy VII Advent Children. Like those in Final Fantasy XII, enemies are integrated into the world environment, but by contrast, battles do not take place in the same field. Instead, there is a short transition and players are transported to a new battle screen, separate from the main playing world. The Active Time Battle (ATB) system returns, but works differently from its predecessors. Users are able to chain large numbers of commands together in order to achieve attack bonuses.
The first battle system was shown during the E3 2006, but it was only a prototype. A new interface was shown in September 2009 by Square Enix in various Japanese magazines and trailers (depicted in the image adjacent). In battle, the player can only control one character at a time out of a party of up to three.
Multiple commands can be stacked into slots per turn and released at the same time to form a combo. The number of command slots increases as the characters grow in strength. These commands include series staples such as Attack, Fire, Blizzard and Cure, as well as new ones such as the non-elemental Ruin and Ruinaga. The difference between XIII's battle system and the ATB gauge is that these commands can still be placed in the slots even though the bar has run out, and the actions will be executed once the required slots are filled up. The game does not make use of MP but introduces "cost points" for each command, which determine how many times the commands can be used per turn. Because magic cannot be used outside of battle, the Hit Points (HP) of the party is completely restored after each battle.
When engaging an enemy, the camera moves to another position and the battle menu appears, making the battle transitions nearly seamless. The after-battle victory screen in XIII holds information such as the time it took to finish the battle, the highest number of combos executed, the number of break attacks and the quality of battle which is determined by a ranking of zero to five stars. The party will be able to purchase new weapons in the game for use in battle.
A "Stagger State" is one of the new features of the Final Fantasy XIII battle system. The enemy becomes staggered when a chain combo has been maintained for a certain period of time on an enemy, filling its stagger bar to maximum. When a high level combo has been achieved, the enemy will glow orange and enter this state for a limited time, during which the player will be able to inflict high amounts of damage and, if the Launch ability has been learned, may knock enemies high into the air, disabling the enemy's moves. The chain bar will gradually deplete during this period; when it becomes empty, the break state ends.
Roles and Paradigms
The "Role" system exists in order to control what abilities are available to characters in battle. Rather than having all abilities available to characters as they are learned, abilities are restricted to certain Roles, similar to the Job system of previous installments. The Japanese version's Roles are Attacker, Blaster, Defender, Enhancer, Jammer, and Healer, while the English localization calls them Commando, Ravager, Sentinel, Synergist, Saboteur and Medic, respectively. Commandos use physical attacks, slowing the rate of stagger decay and striking for high damage, especially with a high stagger percentage, while Ravagers use elemental abilities like Blizzard and Thundaga, greatly increasing the stagger bar. Synergists use buffs like Shell and Brave to enhance the party's characters, while Saboteurs use debuffs like DeShell and Bio to hinder the enemy. Sentinels provoke enemies, absorb enemy attacks and counterattack, while Medics exclusively heal, using abilities like Cura, Raise, and Esuna. Roles, with a specific purpose in mind for each, are the only means by which to direct ally AI.
Each character may only take on one role at a time, and each combination of three Roles for a given party is called a Paradigm. Up to six Paradigm combinations may be stored in a Paradigm Deck at any one time. During battle, players may switch between them on-the-fly, called a Paradigm Shift. Timing Paradigm Shifts correctly can fill the ATB gauges, making switching frequently in battle an advantage."