Chaos is the Level 4 Limit Break for Vincent in Final Fantasy VII. Unlike most other party members, Vincent only has one Limit per Limit Level, but each transformation has two attacks. Transforming makes him stronger and fully restores him, but the downside is that he becomes uncontrollable. Chaos transforms Vincent into a a winged demon that has an Instant Death attack at his disposal.
Though originally only appearing as Vincent's ultimate Limit Break, Chaos has since become a storyline element in the Final Fantasy VII series.
I'm becoming less human...Vincent
The player must enter the waterfall cave near Nibelheim with Vincent and Cloud in the party and witness the scene, then leave the cave and fight ten random encounters and return to find the Chaos manual on the altar. To get to the cave, the player can either use the submarine, or get there faster via either the green, black, or gold Chocobo obtained through breeding.
Transforming into Chaos doubles Vincent's Defense and MDefense. While in Chaos form, Vincent will attack with either Chaos Saber or Satan Slam, and cannot be controlled. He stays that way until the battle is over, or he falls into Death status. During his transformation, Vincent is immune to Berserk, Frog, and Confusion. The offensive properties of Elemental or Added Effect do not carry over to his Chaos form, but defensive Materia set-ups continue to work. Vincent's Limit Break forms behave oddly under All Lucky 7s since he seems to revert to using his gun though there is no animation for it.
Chaos Saber inflicts physical damage equal to 21⁄2 to all enemies. He uses it 70% of the time.
Satan Slam (also known as Stan Slam in the 1997 US PlayStation release) instantly kills all enemies, and any enemy immune to Instant Death will instead take non-elemental damage equal to 55⁄8 times the power of Vincent's normal attack.
Chaos is Vincent's best transformation, as he never wastes a turn to perform something ineffective. It also makes him much more durable, doubling his defensive stats. As Chaos is much better than the preceding Hellmasker, it is worth upgrading him the soonest after learning that form.
Chaos refers to the void state preceding the creation of the universe in Greek creation myths, or to the initial "gap" created by the original separation of heaven and earth. Chaos also refers to a state of disorder, disharmony, dissonance, lawlessness, and unpredictability.