https://www.khinsider.com/news/Dark-Road-Chapter-3-The-Purpose-of-the-Journey-17896
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Heartless are masses of Darkness resulting from Hearts negativity feeding the Darkness yet being unable to contain it!
Hearts are naturally capable of producing Darkness. No doubt a result of infusing the Darkness of the Heartless of the Book of Prophecies into the Power Bangle.
Eraqus of course ignores Odin’s words and fails to realize what is going on going on a Anti-Darkness obsessed rant.
Unversed are another anomaly. Vanitas is a Pure Darkness that infested Ventus and as such when removed with the Darkened part of Ventus’s Heart it wasn’t the Darkness that couldn’t be contained but the Sin which burst out as Unversed!
Normal Hearts attempt to contain Darkness as a result of an Autoimmune reaction due to their origins starting in the Pure Light. Hearts of Pure Darkness infused with the remains of a Heartless attempt to badly contain Sin!
Riku training in Darkness made his Heart no longer a prison for it. Vanitas’s Heart(which being Darkness feeds on Sin in the same way Spirits feed on Nightmares) alas is still a prison for the Sin that his Heart consumes rather then adapting to the Sin like Riku’s Heart adapted to Darkness!
Kingdom Hearts replaced Darkness as the force of Evil with Sin which is represented by Unversed and Nightmares(Dream Eater Hybrids of Sin and Darkness)!
The Foretellers are Lights consumed by Sin yet has no Heartless remains in their Hearts due to not using the Power Bangle. Of course the moment their Light is convinced that their Negativity is a foreign anomaly they will start spawning Unversed despite being Pure Light!