The world of Final Fantasy XIII is based on a advenced technology civilisation which have strong link with magical Crystals. In ancient times people created Cocoon according to the plan the Farsi gave them. The Farsi are creatures who can listen to the Crystals will and uses the Lusi to accomplish them. The floating city Cocoon is supposed to protect the inhabitants from the 'infection' of Pulse the city which lays below Cocoon.