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".