Kingdom Hearts did use to be this big source of power and light/wisdom that after the events of the first keyblade war got destroyed/swallowed by the darkness ---- with how long it’s been there, any iteration of it now should be like a twisted , tainted, poisoned version corrupted by this darkness; so think a benign / neutral power that has over the years turned very much Eldritch

 Like any Eldritch Entity, digging too deep into the subject of the Entity ( in this case KH and even the heart’s power in general ) will eventually turn you mad. The more insight and wisdom you gain, the bigger the hold it will have on you in the end.

Heartless and Nobodies are cryptids with stronger ties to KH than those who still retain their humanity ( for obvious reasons ).  

They cannot be killed ( unless by Keyblade, seeing as most if not all Light Keyblades contain elements of the original Kingdom Hearts ), do not need to sleep, to breathe or to eat. Their blood isn’t blood but basically dark ooze ( think condensed Darkness ) that will sometimes show an uh, will of its own.  It tends to corrupt / kill any living thing it touches. 

The Humanoid looking Nobodies are still the strongest ---- they have extra special powers ( like Axel’s relationship with the fire element ) and control groups of lesser Nobodies.  If by any chance they get hurt, they also eat these lesser Nobodies to heal.