 In Final Fantasy XII , a mysterious phenomenon known as " Mist " is the key energy which allows characters to cast summoning magic and perform " Quickenings " . After defeating an Esper in combat , the player will be able to summon it to the battlefield . Similar to Final Fantasy X , the summoned creatures become active participants in battle , as opposed to the cinematic attacks seen in previous games in the series . Unlike Final Fantasy X , however , Espers follow hidden gambits , rather than the player 's direct command . The summoner remains an active member in the fight , able to attack and cast support magic , instead of leaving the party or standing idle while the summoned creature fights . An Esper will leave the battle if either the summoner or itself is knocked out , its time limit expires , or it executes its special attack . Some Espers have origins in Final Fantasy Tactics and Final Fantasy Tactics Advance and others are derived from the final bosses of previous Final Fantasy games such as Chaos , the final boss of the first Final Fantasy , and Zeromus , the final boss of Final Fantasy IV . 
