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Chaos is the overarching antagonist of Castlevania: Aria of Sorrow and the main continuity of the franchise as whole.
It is the Evil Counterpart of God himself, the essence of evil that was incarnated in Dracula and made him the King of the Night. After his final demise in 1999, Chaos sought a new vessel in Dracula's reincarnation Soma Cruz. While Soma managed to defeat it and refuse his destiny, Chaos cannot be killed permanently and will find a new Dark Lord eventually.
Chaos is the name given to the mysterious Cosmic Entity who rules the Chaotic Realm. Very little is known about it and its nature led to much fan speculation. Even the characters can only theorize about its exact nature, with Olrox experimenting to find out.
Its entry in the Enemy Data (written out when the game was released) describes it as the root of all chaotic things. It is stated that every demon up to Death himself are bound to it; and that it is linked to humanity's evil, being as much as its source and a catalyst sustaining itself on it.
Chaos is the force that makes Dracula into the King of the Night: the absolute ruler of Darkness and Evil, who controls every monster and corrupts the heart of humans. It most probably shares, and even inspires, his goal to plunge the world in darkness. The King of the Night being Evil Incarnate, this means that Chaos is Evil itself.
The sequel Castlevania: Dawn of Sorrow explains that the King of the Night is necessary for the Balance Between Good and Evil, as one cannot fully exist without the other. With the King of the Night being in direct opposition to the work of God himself, this makes Chaos the Anti-God, his Evil Counterpart nearly equal in power and influence. To cut a long story short, it is the sum of every flaw of creation, the indirect source of everything wrong and every woe that befalls the world, likely born from the negativity that festers in the Chaotic Realm.
Whether or not Chaos chooses the King of the Night and controls his actions is never explained; but both games imply that Dracula can only be succeeded by his reincarnation, who can then assume the same demon form. The title Evil Incarnate suggests that Chaos is not a superior power but that Chaos and Dracula are one and the same.
It can be assumed that Chaos is bound to the Chaotic Realm and merely fulfils a cosmic role, needing the King of the Night as an avatar to influence the Physical World. Chaos was apparently personified as Dracula as long as the King of the Night existed, and was cast back into the Chaotic Realm after his final destruction.
The Chaotic Realm
Just as its enigmatic ruler, the Chaotic Realm is never fully described. It is a warped dimension where negative energy and human malevolence gathers, which only Count Dracula or his reincarnation can enter. It exists outside of the time-space continuum, as a pitch-black dark void, hyper-speeding at random on an erratic course. It looks like a creepy mash-up of different locations, all upside-down or reversed and creepily monochrome.
The Chaotic Realm is likely where Dracula and the Castlevania are sent when destroyed, for it contains a merger of all parts of the Castle. It might also be linked to Hell and the Abyss where monsters come from, as it spawns many.
Chaos is clearly sentient, but it has no dialogue and does not display any personality or sapience, apparently acting more out of instinct than anything else. As such, its desire and motives remain unexplained.
Chaos is an Eldritch Abomination, almost impossible to describe properly. It first appears closed like a shell covered with limb-like vines looking like carved stone; and sports three figures shaped like winged-humanoid statues holding a glowing orb or different color.
Chaos' true form appears as a stone-like symmetrical shape with no beginning or end, surrounding a Dark Core of changing colour from which protrudes a structure of bone-like appendages with no end in sight (and possibly none at all) basked in a pale red light.
Chaos' picture for the Enemy Data looks nothing like how it appears. It depicts several grey, stone-like, floating diamond-shaped figures in a somehow pyramidal arrangement, with two floating outside and shapeless white masses of energy below the “pyramid”, that seem to emerge from the diamond-figures.
The game takes place in 2035 during the solar eclipse that is to happen in Japan. Soma Cruz and his childhood friend Mina Hakuba are dragged into the Castlevania (which was sealed in a solar eclipse after Dracula's final demise). Soma must now find an exit before the Castle's evil influence proves fatal to Mina. He discovers that he wields Dracula's power of Dominance, absorbing the souls of every monster he kills to gain their powers.
Soma meets the demented missionary Graham Jones, who believes himself to be Dracula's reincarnation, fated to become the new King of the Night, but it is revealed after Soma kills him in self-defense, that Dracula's real reincarnation is Soma himself.
Soma manages to resist Chaos' influence, but the only way to reject his legacy is to venture in the Chaotic Realm and confront the demonic entity itself. Soma manages to defeat the Anti-God itself, but Chaos can never be truly destroyed and is bound to find a new personification sooner or later. The sequel Dawn of Sorrow focuses on the rise of the King of the Night, and the fact that an incarnation of Chaos is somehow necessary for the universe.
Right when the battle begins, Chaos absorbs all the souls in Soma's grasp and renders him almost powerless. Soma must strike the three statue-figures on the shell. The figure holding a red orb attacks with three diamond-shaped projectiles. Destroying it restores Soma's Bullet Souls and his ability to use attack spells. The figure holding a blue orb attacks by turning into a gargoyle-like creature and rushing towards Soma. Destroying it restores Soma's Guardian Souls and his ability to summon Familiars. The figure holding a yellow orb conjures a rune that drains Soma's magic power. Destroying it restores Soma's Enchanted Souls and his ability to boosts himself.
When the three figures are destroyed, Chaos "opens" itself and the battle becomes much harder. Soma must target Chaos' core but he must first destroy the four eyes in the corners to lower the entity's defenses. The Core is protected by a huge Skeletal Dragon, who gravitates around it to shield it from Soma's attacks dealing huge damage on contact and who can fire a green bouncing ball. Chaos attacks by firing a circular wave of needles counter-clockwise, and by conjuring four small green orbs that fly slowly towards Soma.
Should Soma lose the final battle, Chaos makes him its new incarnation and anoints him as the new King of the Night, restarting the never-ending war between good and evil.