Chaos is the name given to the mysterious entity who rules the Chaotic Realm: a demonic and warped dimensional plane where negative energy and human malevolence gathers, which only Dracula or his reincarnation can enter. The Chaotic Realm is likely the place from where Count Dracula and the Castlevania emerge when resurrected, given that it contains an amalgamation of all parts of the Castle. It might also be linked to Hell and the Demonic Planes where the demons and monsters come from, as it spawns many monsters.
Very little is known about Chaos and its nature led to much fan speculation. Its entry in the Enemy Data (which was written out when the game was released) states that it is the root of all chaotic things. This means that it is the Source of All Evil, and the anti-God of the Castlevania universe; likely born from the negativity that festers in the Chaotic Realm.
Chaos is the force that makes Dracula into the King of the Night: the antithesis of God described in Dawn of Sorrow to be necessary in the balance between Good and Evil. It is clearly sentient but has no dialogue and does not display any personality or sapience, apparently acting more out of instinct than anything else. Whether or not Chaos chooses the King of the Night and controls his actions is never explained; but Aria of Sorrow and especially Dawn of Sorrow imply that only Dracula's reincarnation can succeed him as the King of the Night. Given that Dracula is described as Evil Incarnate, it seems 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 that it merely fulfils a cosmic role, needing the King of the Night as an avatar to influence the Physical World. Chaos was apparently was personified as Dracula as long as the King of the Night existed, and was cast back into the Chaotic Realm after his final destruction.
Chaos itself is an Eldritch Abomination impossible to describe properly. It first appears closed like a shell covered with limb-like vines looking like carved statues; and sports three figures shaped like winged-humanoids holding a glowing orb. Chaos' true form appears as a stone-like symmetrical shape with no beginning or end, surrounding a Dark Core of changing color from which protrudes a seemingly endless structure of bone-like appendages basked in a pale red light.
Chaos' picture for the Enemy Data looks nothing like how it appears when met in person. 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.
Aria of Sorrow
The game takes place in 2035 during the solar eclipse that is to happen this year in Japan. The primary protagonist Soma Cruz and his childhood friend Mina Hakuba are dragged into the Castlevania (which was sealed in the darkness of the solar eclipses after Dracula's final demise), and Soma must set out to find an exit before the Castle's influence proves fatal to Mina. In his quest, he discovers that he wields Dracula's power of Dominance, which enables him to absorb the souls of the monsters he kills to gain their powers.
Soma eventually meets the demented missionary Graham Jones and learns that the reincarnation of Dracula is about to become the new King of the Night. Graham believes himself to be Dracula's reincarnation but it is revealed after he fails to kill Soma that Dracula's real reincarnation is Soma himself. Soma manages to resist Chaos' influence, but the only way to avoid becoming the new King of the Night is to venture in the Chaotic Realm and confront the demonic entity itself.
Soma ultimately manages to reject his Dark Legacy, but Chaos cannot 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.
Should Soma lose the final battle, it triggers a bad ending during which Chaos incarnates himself within his body and makes him the new King of the Night, restarting the never-ending war between Good and Evil.
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.