Here we go.Alright look this is about kingdom hearts and PS.there will be no disney stuff at all!Instead wholley original stuff revioving around PS style heartless and nobodies(and the key blades)All right?Good.
About ten milenea ago a race known as the chasers mastered the ability of unlocking latenet potential in a person's soul.This led to them assuming an almost god like role for millenium.For unknown reasons a race from the far realms attacked these beings creating a long drawn out war and in the process killing the chaser's leader turning him into a heartless and nobody.these two beings are cast apart across the planes foever wondering.In the meantime a failed far realms invasion splits the chasers' prime into two worlds a world filled w/heartless stuck in the plane of shadow and a world inhabited by nobodies in the plane of radiance.The leader's nobody and heartless in their travels seeded slect bloodlines w/a fraction of the chaser's power to unlock the heart and also forged the tools needed to do so.when these two met at some time they vanished in a wave of pure magical energy which creadted a portal from the shadow plane to a small prime.In response the heartless attacked this prime turning millions into heartless and at that same time trigerring the blood lines thousands are now being drawn to the primes to face the darkness.They do not know why or when they will face it all they know is they will.
I can't say Ive ever played kingdom hearts, but there are a few... oddities in your story.
1: a Far Realms race. The process of a breach between the multiverse and the far realms is so wonderfully complex that it, thankfully, almost never happens. I think the last time it happened we wound up with Illithids who took over nearly everything. There's a reason Illithids are in so many campaign settings. The breach required to bring through an army would be catastrophic, and would have shaken the planes in a number of other ways. It would not be an isolated incident.
2:All these heartless trappen in the Plane of Shadow. I would change this to some demi-planar prison. Why? The PoS gets plenty of visitors every day. All those shadow-dancers and primes looking to use it to get to other primes, The Heartless would need a reason not to have escaped before now.
3: I don't know what a Nobody is, or what a Heartless is, but you said that the Kind Heartless and the King Nobody both seeded people with a bit of their power (why? and how?) and created the means to do this through items. Are these the Keyblades you mentioned? I thought those were weapons gainst the Heartless. In which case the King Heartless making them is kinda weird. Unless it can hurt the other Heartless BECAUSE it was made by the King Heartless. In whiach case he still needs a valid reason for making them.
Thats all. I hope I dont sound too negative, but these didnt make sense to me.